I get it sometimes. I remember reading about it- what you're currently doing is supposed to go to the part of your brain that stores your short term memories, then to the part that stores the long term memories. Apparently sometime (when deja-vu happens) what you are currently doing skips the part of your brain that stores the short term memories and goes right to the part that stores the long term memories and it gives you the feeling you've been there and done that before.