Thread bunching and knotting under fabric

Question

I will be sewing along beautifully then all of a sudden I'll hear a different noise, the thread breaks and I have about 2 yards of thread bunched up and tangled on the bottom of the work. 
If I watch the bobbin case while sewing VERY slowly I can see the thread coming to the outside of the bobbin case and crossing over it!

Is my tension off? How do I fix this?