Chads93GT wrote: Are rock slides/collapses in cave diving a frequent occurance or is this a fluke chance, much like a breakdown collapse in a regular cave?
It's rare, but it happens, especially as a result of attempting to squeeze through an unstable restriction. It may be collapse of the passage itself, but more commonly a slope of other material such as sand or gravel forms part of the restriction and when part of this is disturbed it slides down to block the restriction. The most famous case was in 1991 (IIRC) in Indian Springs in Florida when Parker Turner drowned. In a few cases water flow through the restriction supports the unstable slope so that when the flow is blocked by a diver going through the restriction the sand/gravel/rockpile slides down on top of him. Not pleasant, especially when exiting.
The presence of his scooter at the restriction sounds like positive news, assuming he left it there as a signal and did not deliberately aim to leave it there as part of his dive plan.
Andy