Imagine two cities with power plants. Each power plant produces 2-4GW of power. There is a power line between cities capable of transmitting 1GW. What is going to happen if one power plant closes one unit producing 500MW for maintenance? Answer: nothing much, since the other plant and transmission line can handle the increased load.
Now imagine that a blowhard greenpeace hippies closed one power plant and installed wind turbines around the one of the cities. What is going to happen if a sudden anticyclone causes a windless weather over a large region? Answer: whoops, your the power line between cities has just melted, even though the other power plant had enough capacity to handle the load.
And this is not a theory, such things might happen any time in Germany now. Its grid is overstressed because renewable energy generation and consumption are quite often not correlated.