Reactor SCRAMS are not terribly uncommon and I believe that most reactors are designed w passive shutdown mechanisms so that if there is a loss of power, control rods are inserted. Power is required to keep the control rods out.
That doesn’t mean that there isn’t plenty of room for incompetence and Fukushima was one of those cases. I don’t know the background on all of the problems but I believe the generators were required to keep the cooling pools containing used fuel rods full and circulating. The generators were in a low point so they were flooded. The cooling pools lost water and the used fuel rods overheated. I don’t know if that led to the reactor problems or was separate.