According to my friend, the freq drop was caused by a sudden large supply surplus over the instantaneous demand. Nuclear plants were offline and there was nothing to absorb the freq drop at that moment.
In the case of the rotating generators, yes. In the case of the solar panels, I do not know: I guess it depends on the inverters characterists? Spanish is not my native language, so I may have mixed it up when talking to him.
Wouldn't a supply surplus cause a frequency increase, not a frequency drop?