I had something similar happen a few times on my PQ1500s when FMQ and each time it was because I forgot to put the presser foot down before starting- with a floating foot, or a hopping foot, it's easy to do. You get some nice stitches for the first inch or so, but then the fact that the tension discs are not engaged and you get the situation you are describing, with big loops, on the bottom.
Rob