I've got a Toshiba Satellite L300 laptop that does precisely the same thing as 10Watt describes. It happens when you're typing, and you inadvertently brush the touchpad with the fleshy part of your palm at the base of your thumb, at the same time as you have a critical function key such as "shift" depressed.
The combination of touching a critical function key and the touchpad at the same time creates a signal to move the typing cursor to another line.
It's infuriating - but there's a fix, and Google has it. It's a simple little program download that disables your touchpad while you're typing text. It seems to work pretty well for most people.
https://code.google.com/p/touchfreeze/downloads/list