Wow. I'm not sure why that happened, unless perhaps the tension between your threads was unbalanced. My thinking is that the top tension was too tight and the bobbin tension too loose, so the bobbin thread caught the minky and the top thread pulled both to the top.
Threads (meaning your top and bobbin machine threads) do tend to balance out after washing, so it may take care of the problem. However, there's no way to know for sure until the quilt is washed. I think a cotton or 80/20 batting is likely to help with this more than a polyester batting, since cotton batting shrinks a little, which in turn pulls on the quilting threads.