It’s just GPS drift. Happens especially when you are inside. Your phone is trying to lock onto that signal, and your avatar seems to be running all over trying to find it, or thinking it is somewhere when it’s really somewhere else.
The game seems to be lagging much more now, with complete drop outs for a few seconds, and then the avatar running about trying to catch up to you. I’m assuming that’s because of increased load on the servers, but I’m not sure if that is because of the December event, the release of the game to India and the rest of the world that didn’t have it, or because of all the additional measures (and programming) added to try and eliminate cheaters.