This is pure speculation on my part.. but this npr story says that Russia controlled the dam and was lowering the reservoirs level since last fall..  Did Russia do this to blunt Ukraines counter offensive?

https://www.npr.org/2023/02/10/1155761686/russia-is-draining-a-massive-ukrainian-reservoir-endangering-a-nuclear-plant

The reservoir is essential to supplying water to otherwise arid farmland in the southern part of the country, according to Brian Kuns, a geographer at the Swedish University of Agricultural Sciences who has studied farming in southern Ukraine. A network of canals leading from the reservoir irrigates roughly 200,000 hectares (494,000 acres) of farmland that is used to grow sunflowers, grain and vegetables. "It's very important locally," Kuns says.

The reservoir was also a critical source of water for the Crimean Peninsula, which is supplied via a 403-kilometer (250 mile) canal. After Russia invaded Crimea in 2014, Ukraine diverted water from the canal, leaving the peninsula parched. Following Russia's larger invasion of Ukraine in February 2022, one of the goals was to restore Crimea's water supply, and Russia did so that summer by diverting water out of the reservoir.

Russia appeared to have spent several months using the Kakhovka Reservoir to refill a network of reservoirs in Crimea, according to David Helms, a retired meteorologist with decades of experience working for the U.S. federal government, most recently at the National Oceanographic and Atmospheric Administration. "There's 23 reservoirs; they're topped off," he says.

Opening the floodgates

Then on Nov. 11, 2022, as Ukrainian forces advanced, Russian troops blew up a road over the Kakhovka Hydroelectric Dam, which controls the water level on the reservoir.