Drought combined with the fact that basically all of America's hydro dams were thrown up within a 30 year time span...and all of them are reaching EOL, as structures, at the same time consequently--and there's no money to systemically rehabilitate or replace the thousand or so presently critical-rated structures now--never mind the thousands of others soon to be in the same status.
Nothing lasts forever, and dam failures will start to become more common, as a lot of those structures were privatized and handed off to groups that just defer maintenance to infinity. And even if they were kept up--man-made rock, AKA concrete, doesn't last forever. Even the genuine article doesn't.