1. Since humans only use about a quarter of our brain power we may be experiencing a sudden increase in our minds capabilities for a short moment resulting in a psychic remembrance of a current situation causing you too feel you have experienced it already.
2. There is a matrix and a glitch has occured. what happens is you experience a repeated situation then you say "didnt this happen already" and then the programmers of the matrix program your mind to say oh its just deja vu.
Jack: I could have sworn I've been here before. But I know I haven't.
Bob: Heh. Deja vu?
Jack. Yeah. What a strange feeling!
It is an illusion of having already experienced something which in actual fact it is being experienced for the first time.
French : déjà, already + vu, seen