I imagine UA made a deal they thought was in their favor. It turned out not to be over time. They hated it. Did it really cost them anything for real, no, but were the flights a cost they saw, yes. As time went on, the cost stood out. New leadership questioned prior leadership's decisions.... "Who cares what those people agreed to 30 years ago, this is nuts! But we are stuck so we might as well make the most of it. But I hate this, so, let's start looking for picky issues in the contract to see if we can get out of this thing. Ah, we got him on X. Cancel!" And maybe from his side, who knows, maybe over time he didn't dot every i or cross every t because he had behaved a certain way for years and he was being feted and celebrated by UA and got surprised when all of a sudden they got picky.

Speculation.