Take a look at the lower two images from BBC [1] and help me understand how a transformer explosion could lead to the generator breaking loose from it's foundation and blowing a huge hole in what looks like one meter thick reinforced concrete? It looks like the generator was rotating when it broke loose, because you can see the windings are all twisted in the same direction. The left hand photo shows that the penstocks were ruptured, blasting water directly into the power gallery and washing debris out onto the roadway.
Could this really be set off by a transformer explosion? Are the transformers in the Generator Hall? Or outside?
I have not had time to research the layout of equipment at Sayano, but often the turbine is below the generator floor. Rather than a transformer explosion, which the BBC reported, it seems more likely to me that the turbine ran away and self destructed taking the penstock with it, or that the penstock ruptured - and an 800 foot tall column of speeding water entered the turbine room, ruptured the concrete floor upward sending the rotating many many ton generator spinning around wildly like a atomic top.
Who's got another quess?
Links:
[1] http://news.bbc.co.uk/2/hi/europe/8206998.stm