I assume you are following the Russian-language media on the recent disaster at the hydroelectric plant in Siberia that killed ~69 people and wrecked the machinery hall, doing a billion dollars of damage.
The English-language media have little real information, and I'm hoping that the Russian media has better information.
As near as I can glean, the feedwater pressure was observed to rise just before the explosion, so I'm guessing that the turbine speed regulator mechanism failed, allowing the turbine to be driven to bursting speed.
Or, the turbine disk developed a crack and just came apart, perhaps triggered by a speed increase that had heretofore caused no problem.
There are also many reports that the powerplant was known to be dangerous because of monumental lack of maintenance.