As far as I know, the units were scrammed as the systems are designed to do so.
One of the units has some problems with its cooling system, which would be the Residual Heat Removal system. The RHR removes decay heat from the core after the plant is shutdown, particularly when the units are depressurized. Even if there is a minor deviation from the expected or anticipated operation, staff will take the precautionary step of alerting the local government of a problem.
Apparently Units 1 and 2 at Fukushima Daiichi (Fukushima Plant 1) have lost power. They should be powered by Emergency Diesel Generators, which should provide power to cooling pumps. It's possible the EDGs and/or pumps were damaged.
In theory, they should be able to use natural circulation.