I get the same thing. I've no idea what causes it. I catch it fairly quickly and pull the fabric at the back, hear the "ping," and carry on. I thought maybe there was a burr on my presser foot, but I can't find one. It doesn't happen with my walking foot, but I use the other feet so rarely, that I don't know if it happens with them. If you should ever find out what causes it, I'd love to know.