At every club he worked at, Mourinho's best came either in the first or second season. He collapsed at Madrid, he collapsed at Chelsea, and it was entirely predictable he would collapse at United, too. He was grumbling throughout the second half of 2017/18, he publicly attacked the players at every opportunity, most notably at the Sevilla press conference. Our style of play was absolutely unwatchably awful, I could barely name a good performance in the calendar year of 2018. Yeah, yeah, "he wasn't backed" - but it all clearly started to turn sour long before the summer of 2018.
So yeah, his usual methods gave us the peak of 2nd. It was only ever going to be downhill from there.