[gku]

TL;DR

> REN said: “Due to extreme temperature variations in the interior of Spain, there were anomalous oscillations in the very high voltage lines (400 kV), a phenomenon known as ‘induced atmospheric vibration’. These oscillations caused synchronisation failures between the electrical systems, leading to successive disturbances across the interconnected European network.”

- Redes Energéticas Nacionais (REN) is a Portuguese energy sector company

[diggan]

Which is one theory, out of many. Still no concrete answer beyond a couple of theories.