I was wondering if you've ever heard of anyone fracturing metacarpal bones of the hand from bowling? I have been bowling off and on for over 30 years, on leagues and in tournaments. I have a fingertip 15-pound Black Widow, fit for my hand for about 6 months now (my beloved fingertip 15-pound Sumo was stolen out of my car). Three weeks ago I was bowling in my league and felt excruciating pain in my bowling hand, fingers and wrist. The injury occurred just past the bottom of the pendulum swing, just starting the upward swing motion. As a result, I dropped the ball and was unable to grip the ball from that point without experiencing pain. Upon x-ray, I was informed that I had two stress fractures, one in each of my 4th and 5th metacarpal (hand) bones. I have researched bowling injuries and have found wrist, finger strain/sprain, and bowler's thumb, but have found no injuries from bowling such as I have experienced.
I can not say I've heard of an injury like what you describe.
I'd suggest that the ball should be falling/rotating off your hand as it swings down. Moving upwards and outwards is not optimum. You see this more with recreational bowlers still gripping onto their house balls (which usually do not fit well).
Sorry to say your injury is the first I've heard of, like you described. Good luck with the healing. Best wishes.