imo I would say the single, biggest cause was the movie. the movie drained his energy and emotions then once he got a bad reception (and rightfully so, the movie was terrible af and felt egotistic) he pretty much just gave up mentally. now hes doing the bare minimum in his new episodes to continue generating $$$. all his new episodes now hes just making the same facial expressions while making poop jokes that feels forced. it's like hes sticking to the same script in every episode
