You have everything 180 degrees backwards. It is a matter of public record that Ukraine had been firing HIMARS missiles at the dam since October in order to "test" the floodgates and assess the feasibility of destroying the dam and unleashing the flood.

From the WaPo article, "Inside the Ukrainian counteroffensive that shocked Putin and reshaped the war" (Dec. 29. 2022):

"[Maj. Gen. Andriy] Kovalchuk considered flooding the river. The Ukrainians, he said, even conducted a test strike with a HIMARS launcher on one of the floodgates at the Nova Kakhovka dam, making three holes in the metal to see if the Dnieper’s water could be raised enough to stymie Russian crossings but not flood nearby villages.

"The test was a success, Kovalchuk said, but the step remained a last resort. He held off."

The easiest and simplest and most plausible explanation here is that the Ukrainians, faced with pressure from their Western benefactors to to do something, and having been obliterated in their attacks on Russian defences , decided it was time for that "last resort".