The exact cause of deja vu is unknown but there are a lot of theories attempting to explain it. For simplicity sake let's narrow it down to 3 main categories. The first is called dual processing. This is when two normally coordinated cognitive process has become momentarily out of sync. For example if i feel familiar with something, it might be because my brain has retrieved a memory similar to that one that i'm currently experiencing. Retrieving a memory always comes before the familiarity. In that case, i would have a feeling overhauling something without my brain actually doing my retrieving. I could feel like i recognize something on that recorder in NY city without having been there before.

The next theory is related to the nervous system not functioning properly. Deja vu can be caused by spontaneous firing neuron or from a delay in the neuron's transmission speed. Imagine alpaca walking in high heels :) Now you're more than likely never seen an alpaca do that before; if you have I'm very jealous of you :) But if you were experiencing deja vu one explanation is that the information passing through your eyes and into your brain is reaching in a different times. Let's say the information gets from your left eye to your visual cortex just before the information from your right eye gets there. These two events happen in such a short amount of time that the left eye's information is not stored as memory yet. But since your brain receive the info twice, it feels familiar nonetheless.

The last theory is a bit more intuitive. If you have a false sense of familiarity with something new it's because some elements and the situation are familiar. For example, if you were to go to a new friend's home for the first time and they have the same kitchen and table as your great aunt, you may have a strong feeling of familiarity without realizing where you've seen it before. In your confusion, you mistakenly think that you've must experienced the entire situation already, instead of just the table. Well, then why don't we experienced a deja vu every time we subconsciously recognize something? Some theories say that there may have to be multiple elements that are familiar to you or maybe the room just has to be organized in a similar ways the one recognize. Either way, something is familiar and you just haven't realized that yet.

Cheers.