They probably promised Pence “when Trump is impeached, you’ll be President” and he knew there was no other way he would ever attain that office.
Pence’s conduct that day always struck me as very odd. You may have the best explanation for it; force Trump out early and you’ll become our President, Mikey.
And Pence came so close to being president twice. Twice he could taste it, twice! And it was snatched away from him. He must’ve been livid. Must be worse for him now, the slow burn.