Try my wild speculation out: Earthquake hits, turbine (aka, the generator part of the installation) shuts down and decouples from the reactor due to excessive vibration. Reactor is still getting power from the grid's remaining generation systems, so not a huge deal. A bit later, the tsunami rolls in and grid power goes down. Computers or the operators command the diesels to start - and they suck in water and hydrolock.
Would also explain why they couldn't get the things back up and running in a hurry after the problem was evident. Hydrolock most engines and they're done.
Something like that probably. 13 engines won't all fail even if they skip a few maintenances.