Like most things in life, probably a bit of everything - shoulder injury, fatigue (lots of basketball over last several years with Olympics and Covid), inexperience/immaturity, his teammates not stepping up to help him.
I think one thing to note though is that Tatum hasn’t had a full offseason is several seasons to work on his game. I expect a jump from him next year after getting a full offseason under his belt to work in his craft. He’s far from perfect, but I do think this is an experience that will drive him to be better. Hopefully the same is true of JB, too.