Without knowing any of the details of their nuke plant, I'm going to speculate that the nuke portion of it wasn't the problem, but more than likely balance-of-plant issues. Not saying that excuses it, but there are a lot of things that can go wrong to cause even a conventional generating unit to have to shut down, much of it having very little to do directly with power generation. I would imaging a nuke plant is even more susceptible in that regard for "abundance of caution" (I hate that phrase) reasons if nothing else.