If you have another back try that and see if the same happens. If it does happen the problem is probably with the camera; if it doesn't happen the problem is probably with the back. I have used Bronicas for many years and I never experience anything like this so can't help further. The cameras and backs are usually pretty robust and seldom give fault. Hopefully someone here will have an idea of what is going on and will be able to help.
Thank yo Neil. I'll do that. My hope is that the issue is not in the body. I'll load up the other 120 back today and see what happens. If I remember right, that back does something similar.