For what it's worth, I was listening to Tip Off with David Locke yesterday, and he said the reason Kanter is not playing as much is that he keeps making mistakes on the court, so it's part frustration and part he is brought to the bench for instruction. Locke said that one time he was pulled last game, he went to the bench where an assistance coach then spent a good deal of time talking with Kanter, presumably to let him know what he was doing wrong and to instruct him.
Yes, I know many of you are not Locke fans, but he's certainly more plugged in than anyone here, and I have no reason to doubt him on this. (Personally, also, I like Locke, and I listen to his Tip Off podcast almost daily.)