I sense the founders used the VC as leverage. They had a closing dinner the night before and shook on it. The VC was reasonable to expect the paperwork to be finalised the next day.
YC changing their mind at the very next day sounds very convenient...
I imagine that everyone's recollections differ. I'd hate to speculate.
Edit: I seem to have touched a nerve, but I don't know what it is. Can someone explain what I said that caused annoyance? I have the usual embarrassing secret obsession with made up internet points.
(There was a radio programme this week that demonstrated the process of giving someone a false memory. I would post a link but First Great Western (the train I'm on) blocks a bunch of BBC website.)
He concluded that he needs to move faster on the big picture, rather than haggling the correct protocol for picking of nits.