There can be other system operation concerns such as stability and spinning reserve. If the intertie to France is lost and 3GW of nuclear power disappears and y

There can be other system operation concerns such as stability and spinning reserve. If the intertie to France is lost and 3GW of nuclear power disappears and you are left with 100% wind then the severed system probably will be out of balance without something ready to go that can quickly. Also yes stopping and starting the machines are often when things go wrong. Things that are stopped tend to stay stopped and things that are running tend to stay running. Keeping a machine hot and spinning avoids thermal cycles and issues with stopping and starting oil pumps, brakes, bearings, etc that can occur.