Not to defend Ronaldo, as he wasn't at the level required to compete at the top (at least relative to his stature and wages), but I think the insubordination may have stemmed from him being unable to accept substandard efforts from our players.
I mean, look at Sancho and co. now. . .
Ronaldo's ego desperately wanted to win 'something' with United, to show the world his importance and be the 'homecoming hero', so to speak. But these lazy players putting in poor levels of effort drove him insane, to the point he lashed out and acted inappropriately.
To reiterate: I'm not defending Ronaldo's behaviour at all. It was clearly unacceptable. But I genuinely believe it stemmed from the lack of application and the low standards running through the dressing room. He just couldn't deal with it and wanted out.
Additionally, there's also the point where he couldn't accept his own level dropping. That will have contributed to his outbursts, sure, but I'm not so sure it was the root cause.