Ukrhydroenergo, Ukraine’s main hydropower generation company, has said that four oblasts will suffer short-term water supply problems due to the destruction of the Kakhovka Hydroelectric Power Plant (HPP) and the spillage of the reservoir, and Crimea will not receive water for at least a year.
From what I’ve seen I believe it to be sheer incompetence. Satellite footage in the days prior to the collapse show turbulent water that could be minor overtopping. Additionally there was only one spillway open closest to the generation house, and the road deck above the spillway can be seen to collapse over several days. I’ve seen some talk that it could potentially have been enough to scour away the soil beneath and compromise the structural integrity of the dam along with the overtopping.