Looks like it may have failed from undermining. Percolation from the earthen dam created voids under the slabs.
I think you underestimate the power of moving water. I think that a lot of other people who are responsible do too.
This is the principal behind "Digger Dams", which I will be using on my hunting property hopefully this spring. Cut and leveled tree trunks, set into the slope, cause the cascading water to dig out the downhill side. This creates ponds. I hope to have 4 or 5 in series in the same gulley. This will privide standing water for game, where there is now only watershed.
Yes, that is probably what made the big hole. But my point was more about how that hole didn't need to be there for hydraulic forces to rip away the concrete skirt once the rebar had failed.
