I was once told (and I dont know how reliable or correct this is...) that when we die, an endorphin is released into the brain, which makes the brain replay our life all over again, and it picks up on random memorable bits due to certain triggers setting off the stimuli... so technically we could be living a replay of our lives right now, and when we get deja vu, it's just our brain remembering the events again.
Like I said, not sure how accurate it is...
