Almost certainly sensible rules for what to do in an emergency. When you first feel the shake you don't know how big the earthquake is so you drop the control rods and shut down the reactor. Then you see how bad it was and whether anything was damaged. If everything is OK then you restart the reactor.
I certainly hope the reactor was built to withstand a much stronger magnitude quake than a 5.9
Actually it is not sensible. The one thing that is designed for an earthquake is a nuclear reactor. Now you have to recover from an earthquake without any power.
And the one thing required to keep a nuclear plant safe is electric power. Now you are dependent upon the grid.
It is actually dumb, dumb, dumb. But folks don't think things through. Before the Tresher accident the first thing a sub did with flooding was scram the reactor. Then the ship sank because of lack of propulsion. The rules changed.