I have the Kenmore sewing machine #16231. When I was sewing the bobbin holder popped out and a bunch of thread bunched together and wrapped around the bobbin and caused bunching of thread on my fabric just on the bottom. I placed the bobbin holder back in but every time I get it threaded it does the same thing. It keeps bunching and wrapping around the bobbin. Why is this and why does the bobbing holder keep popping out?

For a comprehensive 5 page illustrated tutorial...Check this solution link, just copy & paste:
http://www.fixya.com/marketplace/servicepage.aspx?srvid=17197&ctgid=164&brdid=1060&prdid=131120&smid=23884&mthid=1&mpsaugid=347d7805-9ce5-42d2-b812-b602b6cf7437

Bunching of thread is 99% a top tension problem. Make sure you thread the top thread correctly. Thread the machine with the presser lever up. Test the tension by lowering the presser lever before threading the eye of the needle, by pulling the thread with your left hand and adjusting the tension with your right hand while slowly pulling downwards. There should be no resistance when the tension dial is on zero, and you should be able to feel more resistance when you increase the tension. Ideal tension is between 3 and 5. When your top tension is correct, your machine should operate correctly. If you feel no resistance with the presser foot down and the tension dial on 5, check the tension disks for fluff or broken thread. When you lift the presser foot, the tension disks should open and you would be able to remove any cotton or fluff. Good Luck