Scientifically, Funkhouser says that the brain unconsciously takes bits of information from an object, place, or person and later will "recollect" this information as if it had already occurred before this situation. The brain only needs about one-third of an object's information to develop its image fully. This information gives the person experiencing the dejá vú episode a false familiarity. Other experts believe it's more related to various disorders, but that's a bit skeptical considering that "normal" people also have these experiences. Some even believe it has something to do with the way our memory works, often forgetting information in the short-term but storing in the long-term so we are more familiar with that situation when it occurs.
No one really knows for sure and not a lot of experts really try to venture too far into this topic because results are never conclusive no matter which angle you look at it from (scientific, psychological, etc.).
No one knows exactly what causes it but I have noticed that I sometimes get it from dreams that I had that I later forgot about.
Dreams as well, even the ones you don't remember.