I've had this happen before and it seems to be caused by humidity or getting dirty. Especially if your hands aren't completely dry or have something on them. I've heard that lotion can have these effects on the trackpad. The behavior is the same as if you hold one finger stationary on the pad and try to move the pointer with a different finger.

The trackpad can self-correct this problem, and you can induce the correction behavior by putting the laptop to sleep, holding a finger on the trackpad, and waking it up, releasing your finger a few moments later. You'll notice that where your finger was has become a "dead" spot. This self-correction resets itself every time you wake from sleep, so often times you can just sleep and wake up if your pointer gets sticky or "springy".