Are you doing FMQ when this happens? My machine will do this when I move the quilt in what is a backward motion for the machine. Usually when moving up and to the left, if I remember right. It has to do with the way a stitch is formed. Sorry I can't give a full explanation, there is one online. Going fast in that direction makes it worse, but sometimes it happens even when I slow down. It usually shreds the thread also.