Bottom (bobbin) thread suddenly loosens

Question

Sewing a straight seam will be going well until the bottom thread suddenly becomes very loose and loopy. The top thread is completely fine. The only clue that this is happening is the spool "jumping" instead of unwinding silently and consistently. I suspect a bobbin problem...I rewound the bobbin, but it happened again resulting in more seams needing to be ripped out. Any ideas about what is happening with my bobbin?