My Janome does that if the bobbin case is not seated perfectly. I take it out, dust it all, and put it back. If not in right, it can snap right up during the tension cycle and jam the machine and break the needle. Ugly!
I have read on one of the Janome websites that there is a special throat plate for some machines which has a kind of metal thing on the back which holds it in place, also that you need to be sure your plate is screwed in good and tight.