It's orders of magnitude harder to destroy a dam from the outside than the inside. The Kakhovka Dam in particular was designed to withstand an external strike from a tactical nuclear warhead. So we can pretty confidently rule out the possibility of Ukraine bombing the dam from afar.
In which case, what's more likely: that a team of Ukrainian saboteurs snuck through the front line, past all the Russians guarding the dam, and set off the internal explosives without anyone noticing, all so they could kill thousands of their own civilians for no tactical gain... or some junior Russian officer got panicky when they heard the counteroffensive was starting and blew the dam early?