This wasn't a sudden problem with the nukes. The first day, a small problem with pumps were reported at one of the reactors. Then 2 days later, another problem, then another and another, just strung out, but all rooted into the basic problem of not having water pumping into the containment structures/fuel pools.
Just don't see why this was allowed to mushroom into a disaster on top of the other disasters that have hit the Japanese.
Perhaps many of those with the ability or knowledge to deal with the plants were dead, injured, trapped, or just missing.
I can see a plant figuring it out if 10% of their personnel aren’t around. But not if 75% can’t be found.