Various factors, really. Not everyone loses form for the same reason. A lot of it is psychological or a bad habit creeping in. A player may have been struggling to move his feet early on in his life, and practiced hard to start moving the feet. However, after time and scoring many runs, he can start focusing on other things such as a perfecting a particular shot, and forget to practice the basics. This can lead to that same problem of moving the feet creeping back.
Another is if despite making no mistakes, the player just gets unlucky 3-4 times and a really good ball just gets him. This weighs in on the player's psyche, and starts affecting his confidence. Confidence, just like every area of life, is paramount in sport. If the player is lower in confidence, he may not play naturally according to his instincts or make a rash decision out of frustration.
The body is always aging and always changing depending on diet and exercise as well. Much of sport is constant adjustment to all these changing factors.
Finally, a player who is used to playing on a particular type of a pitch may take time to adjust to another type of a pitch in the other side of the world. This adjustment period may yield low runs and affect his confidence and mindset.
And last but not least, let us not forget that these are human beings. They have personal lives. Any kind of problem or stress there may affect them.