I think this happened to an extent too.
I think when they started going public, it became untenable, and they chose to do that and I think it was purposeful, my understanding is it was after many private attempts.
I also think some donors may have consciously decided not to donate to force him out, but I think others probably just thought he was doomed and it was a waste of money.
In the end, if he remained and was nominated, I expected to see a commercial 2 minutes after his acceptance speech listing off all of the Democrats in his party that said he wasn't fit to be president. I don't know how he can recover from that.