It could be your thumb accidentally touching the bottom of the screen when you're scrolling with the trackpad. I found myself doing that all the time so now I just scroll by using the touchscreen instead.
I noticed the sensitive trackpad issue on all three 9930's I tried in-store. It's great if the os update fixes it, but why should that even have to be dealt with on a $509+tax device? The vast majority of people buying these don't hang out in CB, so they would be both clueless and pissed. I wonder what the in store return rate is.