Looking at the photos it seems that the antifoul around the keel area flaked off.
Possible sign of flexure near the keel before it broke off?
Good point, I had seen the bottom paint flaked off around the keel. You may be right that there was flexure near the keel before the failure which led to the paint failure.
I actually has blown up the close up photo trying to get a better sense of what could have caused this. When I did there were a few more oddities visible. For example if the failure resulted from a lot of flexing of the hull, you would expect to see a lot of spider cracks in the gelcoat and barrier coat, I don't see those.
When you blow the picture up, it looks like you can see the second and third from the stern keel bolt washers in place and intact. That would suggest that this was in part a keel bolt failure, but if this was a keel bolt failure, then what would have caused the hull to tear away.
The shoal draft version of this boat had an iron keel with iron keel bolts, and perhaps they rusted and dropped the keel, but I still have to wonder what then would have caused the comparatively clean cut that is visible in the picture.
It will be interesting to see what is figured out. Anyone know where this occurred and what happened to cause the keel to separate?