What causes and how to fix lifting floorboard?
Wood moves over time, no real way around that. The board could have been fitted so tightly originally, that over time it expanded just enough to force it up.

As already suggested, you could trying smacking it down with a piece of scrap wood and hammer.  If that works, there are special screws you can use to screw the board down more permanently. The screw head is designed to break off to make as little of an appearance in the board as possible.