Standard safety precaution, I would assume. I'm guessing the plant's seismic sensors aren't aware of the severity of the problem, they just are programmed to shut down the reactor if they sense anything.
Just like the smoke detectors in my house go off every time I try to microwave popcorn. An overreaction? Yes, but standard safety procedure for said smoke detector. ;-)
Well, except that, from the sounds of it, the plant shut-down was a human decision, not as a result of computer.