I'm pretty suspicious generally; maybe it's human error, but high level malfeasance could figure in.

that it was a rare phenomenon caused by large temperature variations in the different parts of the countries. These variations altered the behavior of transmission equipment causing frequency variations which triggered shutdowns in the equipment (running generators at the wrong frequency will damage them severely).

Outside of the usual conspiracy forums I see no evidence of malicious intent. It did highlight how vulnerable grids are to temperature extremes (like those caused by global warming) and how modern civilization has ZERO backup plans for the loss of electricity, which is absolutely CRITICAL to every day life. Loss of power for more than a day starts unraveling our society at all levels.

This is why rooftop solar and battery backup should be REQUIRED on all new construction.

Generator frequencies aren't in the least affected by ambient temp differentials across the grid.

Absurd speculation

the temps of transmission lines and sub-stations. Temp can change resistance and operating parameters, resulting in frequency fluctuations. If frequency varies too much, the system will cut out, which then can trip generators to shut down so as not to damage the grid.

It is hardly "absurd speculation".

https://www.sciencedirect.com/science/article/abs/pii/S0195925523002470

"Ambient temperature differentials in a power grid refer to the varying temperatures across different locations within the electrical grid, which can impact the performance of power grid equipment and operations. These differences in temperature can result from various factors, including seasonal changes, geographical location, and local environmental condition."
"Impact on Grid Operations:
Equipment Performance:
Power grid equipment, such as transformers, cables, and generators, has specific operating temperature ranges. Excessive temperature differentials can affect their efficiency, reliability, and lifespan."

Thanks