From what I've heard the scienfic reason is: It hasn't happened before. It's some kinda miss with signals to the brain and stuff...
I don't believe that tho, I've had deja vu of a deja vu of a deja vu... When it reaches that point it gets creepy as hell >_>'
Well, I think it's some kinda... like, you do a thing you've done before, or something that triggers the memory and the brain one way or another makes re-happen....
I don't like them x__x deja vu's are scary T__T