https://www.nytimes.com/interactive/2023/06/16/world/europe/ukraine-kakhovka-dam-collapse.html
It suggests the most likely explanation is an internal explosion. As you can see there is a maintenance passageway through the large concrete foundation of the dam. This is likely where explosives were placed. Russia controlled the dam and this passageway only has two entrances, so it would be incredibly difficult for anyone but the Russians to pull it off.
This dam was built to withstand external missile strikes, so an external attack would be insufficient to destroy the dam. Something which the US government has ruled out. Likewise despite damage to the roadway on top of the dam and water overflowing the sluice gates, maintenance failure is unlikely as the dam was designed with a concrete apron downstream on top of the riverbed. As such a failure like we saw would require the concrete foundation to be compromised.
This week as the waters receded we got our first evidence that the concrete foundation was indeed destroyed. Experts interviewed by the Times say this is unlikely to have occurred on its own.