Typically thread loops pulling like this are a sign of a massive thread tension issue with your sewing machine. It could also be a sign that you forgot to drop your foot before starting to quilt.
With a modern home sewing machine – if the foot is not in the down position, your top thread will not be under tension, and ugly stitches and big loops of thread will follow soon after.
But notice that the loops of bobbin
thread are only showing up in specific spaces.
The tension issue is not consistent. Notice that it seems to be happening in the same shape - the same curve repeatedly in this quilt sandwich.
This is a key to troubleshooting this free motion quilting issue - taking a close look at the stitches and figuring out what is the actual culprit.