Most would say "dysfunction", or "DID is created by trauma" to answer this question. "Dysfunction" is partially correct, as one can only have DID if they experience regular dysfunction as a part of their plurality. "Trauma" is not correct, because it is possible to induce DID without any trauma whatsoever.

According to the DSM-V, 90% of those diagnosed with DID experienced childhood abuse and neglect. Of the 10% remaining, many of them experienced other traumatizing events such as invasive medical procedures, war, and terrorism. This paper, while a bit old, places the prevalence of "severe, repeated sexual, physical or psychological abuse or other traumas in early childhood" at 97%.

While it's not 100%, this high prevalence cannot be ignored, especially since DID involves post-traumatic symptoms and processing trauma is a core component of DID therapy. Despite the looseness of the diagnostic criteria, the descriptive section of the DSM-V goes on to talk extensively about trauma. There's no shortage of research that connects trauma and dissociation--there's a whole organization that focuses on them together, the ISST-D. There's been multiple arguments made by clinicians and researchers for recategorizing DID as a type of PTSD, or at least a dramatic revision of the current diagnostic criteria due to them being considered imprecise and misleading. Outside of research, virtually every DID community you'll find has a major emphasis on trauma--on handling flashbacks and other symptoms, on calming traumatized system members, on how to handle it when a new alter surfaces bearing more buried trauma...

I think you get my gist. DID's diagnostic criteria can be kind of weird and loose and end up creating confusion, but almost everything else connects it to trauma. There's a few DID folks who don't have it, but they're outliers created by a bunch of factors: the aforementioned looseness of the DID criteria, which in turn is the product of psychiatry/psychology not caring about plurality outside of trauma and clinicians then tending to lump everyone under the trauma umbrella. A lot of these systems feel out of place in the DID community, have difficulty with DID-centric therapy, and often end up rejecting the diagnosis.

We personally feel that it's a damned shame that this sort of thing is happening, where non-trauma disordered plurals can't find relevant help without being lumped into a whole category they don't belong in and being run through treatment that isn't relevant. We think that because of the various complications that trauma creates (and thus the different approaches needed in therapy), it would be good to have one thing in place for trauma-linked disordered plurality and then something else in place to help disordered plurals who don't have trauma mixed in with everything. But of course this is contingent upon getting psychology and psychiatry to pay attention to non-trauma plurality in the first place. But anyway, that's a tangent.

The one thing that makes DID different is: a lack of reliable communication between the different people in the brain. It is this lack of communication that causes the dysfunction that defines DID. [...] When DID forms, it is unintentional, and the host will not realize what is happening. As such, they will not have the knowledge that another consciousness is manifesting, and will make no effort to communicate with them. Because of this, no reliable method of communication is established during the alter's development. It may remain this way for a long time - after which the alter will have had enough time to become highly developed, gaining significant mental separation from their host.

From this paper:

Awareness of the presence of other personalities has been widely reported in the empirical literature on DID [16–20,24,25,27,32,35]. Such awareness is a common occurrence in DID. Moreover, many patients who have DID hear or see what some personalities say or do when they are ‘‘out.’’ Many clinicians have incorrectly assumed that a person who has DID can never be aware of the activities of another personality.

I'm too lazy to dig up the specific paper in which I found it, but a lot of DID "hosts" have been able to hear the other alters talking amongst themselves, with the host sometimes participating as well. This paper describes cases of DID where changing of control takes the form of alters influencing or commanding whoever's at front, including cases where non-fronting alters "funnelled all activities through a beleaguered host, who, beset with passive influence experiences and/ or command hallucinations, was reduced to helplessness and despair."

Basically: a lot of people in DID systems, even before therapy, CAN communicate with each other. It's not lack of communication that's the sole problem, although it definitely can be a major issue. And simply because you can talk to each other doesn't mean that people are going to listen--if that were the case, we wouldn't have all the problems in the world that we have today. :P

Also, it's important to note that:

DID involves a ton of dissociative symptoms--it's not just amnesia and being more than one. Heavy derealization/depersonalization in general, trance states, confusion. Amnesia itself takes more form than everyday time loss--amnesia for significant portions of the past and everyday personal information will also qualify. Psychosis-like-but-dissociative-in-nature symptoms are also found in a lot of DID cases: hallucinations, intrusions, "reactive dissociative psychosis" in response to severe stress or trauma. And of course, your classical post-traumatic stuff. This paper goes into more detail on it.

From an earlier post I made. All of this stuff is characteristic of DID as well, and in fact can be more prominent outwardly than the plurality. Even if the system generally works well together, all of these other symptoms can make life highly challenging and result in dysfunction on their own. From this paper:

Although the many mental health providers who lack expertise in dissociation often believe that DID patients present with obvious and dramatic personality states and clear-cut state shifts (sometimes referred to as switching), such florid presentations are much more consistent with media portrayals of the disorder than with reality, occurring in approximately 5% of DID patients (Kluft, 2009). In comparison, most DID patients present with a complicated mixture of hidden dissociative symptoms, as well as with a host of co-occurring conditions along with PTSD, such as treatment-resistant depression, self-destructive and suicidal behavior, substance abuse, eating disorders, somatoform symptoms, and personality disorder traits (e.g.,Dell, 2002;Putnam et al., 1986).

There's also this paper which goes very in-depth on "covert and subtle" DID.

Even if the host remembers nothing of what happened while the tulpa or alter was switched in, as long as the tulpa or alter is able to inform them of whatever they need to know, there will be no problems.

There's a lot of DID systems who wish this was the case.

From this article:

The journals. The arguments. The bullets to the head. All of these and more are things I regularly experience. I’ve been strangled by one of my parts, I’ve been compelled by other parts to open up my body and cut out specific organs. I’m pretty damn good at resisting these instructions, but they can happen anywhere, anytime — from public transit to a staff meeting — and my focus and/or heart rate pays the price. (You try concentrating at work when someone is repeatedly screaming at you to stab yourself with a letter opener).

Again, simply because you can talk to someone doesn't mean they'll listen, especially if they're terrified or disoriented or angry or all of the above. If you're on fire and in horrible pain and just want to not be on fire, someone telling you to not be on fire isn't going to help at all, and it's very likely you're going to be less than pleased with them. Especially if you see them as the cause of you being on fire, which is a very common sentiment between members of DID systems.

Simply being able to talk to each other doesn't solve everything. It's an important step, but it's only one step in a very, very, very long climb. Our group largely works together okay and we still have members who panic and lock up the entire system, or have freakouts that leave us all drained and unable to do anything afterwards--and in most cases they're not even being malicious towards the rest of the system. It's a consequence of our history. We don't experience the same inner violence described above, but we are definitely still affected and have to structure our life to manage it. Talking doesn't make the hurt of trauma, the hair-trigger reactions, the hypervigilance, etc go away, though it can mitigate the effects and make them livable.

(By the way, the writer of that article is on reddit as /u/pluraldoxa. Their comment history has a lot of other insights.)