To make it short, this is all caused by time travellers from our future going backwards in time to our past and mucking with events. This causes us to sometimes shift from one time line to another. Either they discover that the things that they mucked with didn't turn out exactly as planned or they just have more mucking to do, and then they do more mucking and occasionally we wind up converging back on the original time line.
The shift off of a time line and back on to the original causes the feeling of Deja Vu. That is, you were already through one point in space and time, then they forced you onto a different time line with some mucking, then they forced you back to a convergence with the original time line. The convergence points are Deja Vu events. You have truly been there more than once.
Exactly why they're doing all of this mucking is highly speculative. Probably there are power and control elements involved and possibly the future is so fouled up that they are desperate to try to set things right again. At least it indicates that something is going on in the future. But it doesn't necessarily prove that humans will make it to the future. Most likely it's another species trying to prevent humans from doing whatever galactic-scale damage they may be destined for, possibly by preventing humans from getting to that point.