No one knows the reasons yet. Some rods dropped. Something went wrong with the rod control system. Maybe an errant trip, maybe something mechanically wore, e

No one knows the reasons yet. Some rods dropped. Something went wrong with the rod control system. Maybe an errant trip, maybe something mechanically wore, etc. When a few rods dropped "accidentally" into the reactor, humans pushed the button to shut it down. Now they figure that out and then turn the reactor back on.