This is a known quirk of Thinkpad trackpoints. Once in a blue moon, the mouse pointer drifts randomly; touching or nudging the trackpoint usually makes it stop. Regardless, it stops doing it on its own after a minute or two.

My old IBM 600X (1998 era) had this issue, as did my 2013 T430 and my current 2017 T470, although it hasn’t happened for a long time.