To quote NASA in the first shuttle disaster, "obviously, a major malfunction.."
Not that way.
When a reactor shuts down, needs some cooling for days. All ten reactors were shut down, however, the cooling systems in the older units failed due to the fact that the tsunami had damaged the auxiliary power plant.
Injecting water is a way of cooling the reactor, however, the steam generated has to go somewhere. If everything else is damaged, one option is to keep it into the containment building. Probably the building could not whitstand the pressure and broke up.
The systems were tackling the problem, but the plant is too old and too damaged.