The same thing happens to me, did you just update your Mac? I've found it's been happening since then, also when i press enter a few times the problem seems to go away, hope that helps :s apparently rebooting helps too.
I dont know why it does it, but i do know how to get rid of it, just use four fingers like your index, middle, ring, and pinky, then tap your middle twice while all the other fingers are on the trackpad and it should go away, it does for me everytime.