It's probably lack of maintenance. Since the drought they probably haven't used the spillway that frequently, if they have turbines for power generation that most of the release is through the pen stocks feeding the turbines.

So when they finally get a big storm (5 year or so) then they have to use the release spillway. But cracks can develop in the spillway slabs, and when water flows over the cracks, a lot of water, it can rip the concrete up. Especially if the concrete was spalling around the joints.

Our infrastructure is old, most surface water impoundment structures like this were built in the 60s'-70s.