The "plan" for Tatum to take the next step is for him to average 8 FT's a game, and to get more easy baskets, at the rim. To do this, they felt he needed to put on more weight to take the pounding, and get stronger to finish around the rim.
One: The plan led to the added bulk we see on his frame (and especially his shoulders). And that is messing with his jumper. You could tell right from game one that his stroke looked just a bit different. His elbow was flaring just a bit on his jumper, and didn't look nearly as smooth as he normally was. This small flaw in his mechanics is throwing his percentage off. This is why his FT percentage is down as well. He's fighting his shooting stroke right now.
Two: Back to the plan.... Trying to get the to the FT line, means he's also over driving. So, he's often driving into crowds. He then tries to "euro"step through those crowds to force his way to the rim. All well and good, but one thing the plan didn't take into account is the increased physical nature of this NBA this season. Those extra bumps and hand checks really bother him because he also has a flawed finishing technique. He always is jumping off one foot, usually while trying to avoid the contact, instead of jumping off two and going strong. (Look back at the dunk over Lebron. He jumped off two feet there and went strong.) Because he goes up "weak", he get's off balance, and as such, he's back to being a terrible finisher at the rim. You can tell he's frustrated because of all the complaining. He's yelling at the refs even when he clearly isn't fouled. I think he's yelling at the "plan" as much as he's yelling at the refs. Its almost like he's saying to Ime, Stevens and Drew, "Look, I'm doing what you guys are telling me, and I can't get a call!"
Three: Loss of confidence. His jumper being off, physical play, the plan not working, Smart calling him out, fans p---ed off, etc... All of this is playing in his head, and he's lost some confidence. This happens to everyone when going through big changes. Look at Tiger when he was making swing changes. It happens, and its a hard thing to over come. Reps and hard work will be the only thing that gets him out of this funk, but he will. Give it the old 20 game rule, and by December, he will start to look like the old Tatum. Hopefully averaging 8 FT a game.