If you do find that you have the wrong ball, make sure that the one you buy has a heavy enough rating. My trailer weights around 6,000 on a heavy day and my ball is rated for 10,000.
As far as the weight distribution bars, they might have prevented the detach.In the end this might have been a blessing in disguise as there was no real damage and nobody was hurt.
edit: I see you already have a 2 5/16 ball. I wonder if you somehow accidentally closed the latch before the ball was up inside it. It would look closed, but obviously it would just pop off on a bump.