Zhang is 41 years old and fights like the chinese George Foreman at this point. He comes in heavy and he is slow. His timing however is pretty good. He caught Wilder with a grazing left hook Wilder didn't see which spun him around. Wilder was out on his feet and took the second hook dropping like a stone. No idea why Wilder came in so lean. He didn't use speed or angles.
I agree with this. Except, looking from a different angle, it wasn't a glancing blow. He hit Wilder pretty flush, not full force but Wilder was coming in and it was a bit more than a glancing blow. It spun Wilder, who was clearly stunned. I don't think he was complaining to the ref. I think he was disoriented and didn't realize he was spun completely around. I think he was wondering wtf happened when Zhang knocked him out.
Also, I think Wilder is as mentally fucked as anything. I think he never really knew what the fuck he was doing but his confidence and his right hand took him far. Now he's lost his confidence, he's a multi millionaire, and he just doesn't want it anymore. Regardless what you think of him he's a guy who made 10 defenses of the heavyweight title. He's not hungry anymore even if he still had his little bit of skill left.