This is some surface level analysis that I'm blaming on it being 6:50 am. In addition to the "out of position" discussion that has been touched on, Grant appears to have regressed dramatically in his decision making. In other words, he's often making dumb plays out there.
Last year he was on the floor against the Raptors in Game 7 because he could be trusted to make the smart play on offense and rotate on defense. This year it's very common to see Grant "try to do too much" and turn it over on offense (horrible passes, failing to catch good passes, being in the wrong spot and causing a teammate to turn it over, airballing floaters, etc.) or simply make bad defensive plays (missing rotations, taking what he thinks are "smart fouls" to stop transitions/layups while ignoring the amount of team fouls, etc.)
The long and short of my point is that he's not playing smart this season. Maybe it's a lack of focus, but at this point the game should be slowing down for him and it clearly isn't.