Nuclear plants of the design mentioned in the article must legally have offsite power to continue operation. As soon as offsite power is lost, the plant is required to shutdown. An emergency shutdown is more paperwork than a planned shutdown such as this.
The reason for this is that in an accident scenario, you would like to rely on offsite power to run your emergency coolant pumps for this particular design.
Newer reactor designs don't have this issue, but this is a pretty economic decision considering an emergency shutdown if/when the offsite power does eventually trip. The grid seems pretty unreliable based on past experience, as the article even notes.