This is a hypothetical explanation because I don’t know how grids work in practice. The faster turbines turn, the more voltage they generate because the rate of

This is a hypothetical explanation because I don’t know how grids work in practice. The faster turbines turn, the more voltage they generate because the rate of change in flux with respect to time increases as per Faraday’s law. This means they can compensate for the voltage drops across the network due to increased current as per P=I*V. I think frequency dropped because there was too much load on the grid. More load produces more impedance in the grid, this means that, without additional power on the turbines, they respond by turning slower. Like having to bike up hill.