Why would he abandon if he was riding for himself? he never did it?
He has a hot head, that is a given, so in his hot head he had a rift against Mas. Whether right or wrong, I don't care. He thought in his hot head that he was being slowed down. Now we know that that was very stupid and false. I even said that. But without Mas, please tell me, why would he abandon? he never did it before and he had no reason, absolutely no reason to do it. He would have probably kept bleeding time for sure.
He was trash in 2019 Vuelta on stage 20 and was trash at the Tour on stage 20. he has some history of collapsing in some late stages. Even had a fight with a fan. He probably dodged a bullet in the Giro for hitting the fan. But soldiered on in all cases. he had no reason to quit. why would he quit? during the Tour this year he was pulled by Movistar because he was trash just riding with no form. But he wanted to make an effort to continue. But it was the right decision by Movistar. However I still fail to see why would he quit if he was riding for himself?
But why exactly should he have a rift against Mas in this specific situation? Mas just followed a move and he didn't until it was too late.
To your question "Why would he abandon if he was riding for himself? he never did it?": You can basically answer it using your remark "he has some history of collapsing in some late stages." Maybe with each such collapsing he is getting more and more frustrated, losing confidence and motivation, such that this time he simply lost his head in the heat of the moment and could no longer take it. He already mentioned that after TdF 2020 he thought about abandoning cycling altogether, so it's perfectly plausible that this new setback at the penultimate day was just too much for him.
Apart from some comments by his wife and trainer, which I take with a grain of salt as they clearly try to redeem his public reputation, we have no information what exactly caused him to stop. It may have to do with Mas, it may have to do with Movistar, but at the same time it could also have to do with himself and no-one else.