I suspect that because the latch that holds them on is worn and able to open enough to release from the frame they can shake free. The roads around here in New York are not very well maintained and the cases can bounce a lot, with the bounce increasing as the latch opens. The last case I lost had the latch holding it locked, but wear in the mechanism was allowing it to open slightly. I thought this would stop the problem as the first one I lost did not have a locking latch.
Of course, all of this is speculation as I have never been able to recover a lost case to see the latch after the case's bid for freedom. By weaving a couple black cable ties through the latch and the hole in the frame, I have been able to prevent any motion between the latch and the frame. Time will tell if the roads around here can defeat this fix.