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.

Overheating isn't exactly right, they got shut down because they couldn't be cooled, but they didn't overheat.

could have been something as minor as some sensor malfunctioning.