I don't know how to explain it, and I haven't a clue as to how it happens.
Then why are you posting here?
Anyway, I always thought that deja vu was just reoccurring dreams. Like how you have tons of dreams in one night, but sometimes you can only remember a couple of them or none at all. Perhaps a person has a dream of a certain situation happening, then that situation happens and sparks a memory of the dream that you had. I don't really believe in premonitions or anything like that, but having a similar experience as one you had in a dream might trigger that deja vu feeling.
It rarely happens to me, but once it does I really don't care. People say I'm a loon when I say that I have deja vu, so I don't even mention it anymore. xD