Every now and the touchpad on my T520 either gets stuck in scroll mode, or just stops responding. As a quickfix (not a solution) one can restart the track pad driver with the following two commands:
modprobe -r psmouse
modprobe psmouse proto=imps
So, I’ve discovered there is Function key ([Fn]+[F8]) that enables/disables the trackpad, which also works well to solve this issue.