Why should separate nations ever NOT have independent grids? Short of natural resources like Niagara, I don't know why that would happen. Even in that case there's enough energy to go around.
Discussion
It’s poor planning and quite possibly the hellbent pursuit of profit. Lack of back-up systems too. I believe there were issues similar to this across the Scottish grid and beyond sometime back too in the UK (I recall a friend who worked in this field recounting this). Scotland cold, other areas warmer. Changes in simple electrical resistance due to temperature variations was another issue. Grid infrastructure and maintenance is quite interesting stuff - shows how reliant upon it we all are…