A few other tips you may not know is doing test swatches on the edge of the quilt where the extra backing and batting are. I lay a square of material on top of this area and every time I change bobbin or advance the quilt I do a quick test to insure the tension is all good.
Another trick to check the backing is to run your fingernails under a finished area of the quilt backing and if it feels smooth you are ok. If you hear popping or cliking sounds as your fingernails hit the thread you have a tension problem and need to adjust.
I also use a mirror and flashlight to check the back.
As for why the thread does this, it is usually thread tension, which improper threading of the machine would be part of. Good luck and keep on practicing. My teacher actually had me reset the tension several times after she purpously knocked it out of proper tension. Talk about a learning experience!
peace