clocks drift in gps-what`s the drifting related to?

http://arxiv.org/abs/0905.3313

... variations in total electron content

http://arxiv.org/abs/1102.0661

... seasonal effects analyzed.

Probably not random, probably systematic, but still under investigation (only two papers at arxiv.org, which is surprising). I doubt it will be a single candidate.

It is called gravitational time dilation.

The stronger the gravitational field, the slower the clocks run according to a distant (zero gravity) observer.

So to someone in outer spac, clocks near the surface of the earth run slower (more gravity) than the clocks further from the earth in a satellite (less gravity).

Hence, for us on earth, clocks in satellites run faster systematically.

The effect is small but important enough that it needs adjustments for GPS etc to work.