My deja vu hypothesis involves the theory of parallel universes. This hypothesis uses the third form of parallel universes which states that their are an infinite number of replicates of our universe occupying the same space and time as our own universe. We cannot see them, however, due to the fact they are located in a different dimension. To proclaim that there are an infinite number of universes in the multiverse, we can safely assume that every possibly outcome has or will be concluded. For instance, in one dimension you aren't yet born, in another you are already dead, in another you are president of the United States, in another you have risen as a world leader and enslaved the world, in another the United States is a third-world country, etc.
Onto the actual hypothesis itself. I believe that when we experience deja vu, we are actually traveling into a separate, but almost identical, dimension and then so quickly traveling back into our own. This causes a moment to be lived "twice" giving the illusion that an individual has experienced the particular event in the past, yet at the same time. It's difficult to say what causes the entrance into another dimension, as it is only a hypothesis. It could be that the dimension temporarily overlapped our current dimension.