Possibly some kind of rare cascade failure where it might have been on the edge for a while, and some small event happened that tripped things, similar to the american northeast blackout in 2003. High demand, plus a power station going offline meant more demand on some interconnects, which shorted on trees and were cut off, putting more load on other lines until the entire system collapsed

Continental Europe Synchronous Area [1]

The whole Europe power grid are somewhat interconnected I wont be surprise if this knock on effect start knocking out other surrounding countries.

[1] https://en.wikipedia.org/wiki/Continental_Europe_Synchronous...

Is that a risk with the small amount of interconnect capacity?

Grids are widely interconnected. Problems on one grid can and do cascade to another.