” and they cannot back the boiler down fast enough.”
The ‘boiler’ can ‘back down’ as fast as the load rejection.
Some nuclear power plants are designed to stay running with a full load rejection. Others, only for a partial load rejection.
When I was Plant Hatch (GE BWR) we had a SCRAM for the reason I stated.
The point is, there are lots of reasons for a scram and not in of itself a reason for panic.