On powerbooks, at least, it is seems to be caused by two different phenomena:
  • a relationship between the internal humidity of the layers that comprise the pad.
  • what your machine was thinking about when it last went into hibernation.
For the first, Apple once released a different patch for the recognition of the action of the pad (phew), what it did was take a more grid like reading, average the values to get the point of the finger, and then use this. This slowed things down.

In the second, throw the machine back into hibernation, wait 30 seconds, turn it back on and it should go away.