The tick of my left index finger

When I was in a cafe typing my book stories, I've got my left index finger started ticking toward the left thumb. The movement was just the tick as the string between the thumb and the index finger seemed to be pulled and that caused the index finger leaning toward the thumb. I don't think I had such tick problem on my left fingers. If I had one like this before, I already filmed and shared. Check out the movement on the video. It seemed to be the electronic harassment to disturb my typing.