59.8 Hz is 0.2 Hz under 60 Hz so this must in America see
https://world-power-plugs.com/img/outlet-voltage-frequency-a...A drop in frequency is a sign that the generation is struggling to keep up with the demand.
The frequency of the grid is
most directly linked to the rotational speed of the turbines. To increase the frequency to 60Hz, whoever (or whatever) is in control of the grid has to send signals to accelerate turbines (= produce more electricity).
This automated system got it wrong and sent signals to keep decelerating turbines even more.
Automated systems "knowing better than humans" - what could possibly go wrong? Ask Boeing 737 MAX ...