Smart people tend to be good at finding smart sounding reasons to justify things they came to support for very 'not smart' reasons.
Self-reporting is a poor way to find out such things.
It could be a combination of perception, momentum, and tipping points.
Before there were a lot of vocal supporters of him and thus social pressure (for that sub-group) to either support Trump, or at the very least not be vocal in your opposition. Apart from the normal social pressure mechanisms, there are career risks picking the wrong fights and in some sad cases in the military, risks of physical harms.
Then things add up. People feel safer to express more limited criticisms of specific things. 'I don't like dems, but attacking McCain again is wrong.' Those criticisms are let to pass, which makes people feel safer to express other ones. Then it's thinkable to not support him. Then it's thinkable to oppose him. Then it's thinkable to express opposition of him. It builds up in a way where the measurements of it show all at once.
Just because it's detectable suddenly does not mean it occurred suddenly. It certainly doesn't make it traceable to one factor or incident.