https://www.vox.com/2018/9/26/17905796/nuclear-war-1983-stan...
In short: Russian early warning system operator sees 5 minuteman ICBMs coming towards them. He does not have time to go check with someone. If they are to retaliate, he has to just launch now. No such thing as "waiting for the president to decide to launch". He didn't. Turned out reflections of the sunlight off clouds set off their sensors.