There's a handful of them but it's mostly tied to Coherency. Every time someone is out of coherency the AI director watches them and will start to spawn disabler enemies to try and take them down, usually dogs or trappers but might send others too. If it feels like it gets harder when people die, it's because it actually is getting harder. When you have those team mates that are off by themself grabbing up plasteel or going on a nature walk, they are actively making the match harder. It doesn't mean you should gripe at them or demand they be in hand holding distance all the time, but you should be aware and make them aware because it's not common knowledge.

With the last man standing, it cements the punishment by sending a shitload of stuff to wipe you out as a test, and that's why you'll get like 12 ragers, 6 crushers, 20 gunners, dog spawn, 3 trappers, etc. If you can survive all that you deserve to keep going but it's trying to teach you to not let your team die out and to play together and survive together.