Without actually seeing the problem for myself, I would hazard a guess at the nipple mouse calibration thing.
On a work Toshiba laptop I had, I tended to keep my finger on the nipple for too long, and it started to wander off on its own, then stopped after a few seconds.
It appears this is the internal self-calibration thing - it thinks the push is coming from the mouse mechanics and tries to be smart about correcting it. I got around the problem by teaching myself to take my finger off the mouse when not actively moving the cursor.
The other culprit is a screen resolution thing. Easier to see a problem with good old CRT, as the display are could easily be set to be beyond the physical bounds of the phosphor - less of an issue with the LCD as the screen res. will still be in view somewhere.
And if you're unlucky enough to have a touch-pad mouse, simple leaning on the damn thing can move the mouse well away. And cordless meece and nearby active mobile phones don't like each other too much on occasion either . . .