I know this sounds nutty, but please don't get me wrong and hear me out. I could actually envision MacElwain doing something like this.
It seemed to me that on most plays, Grier looked at only one player - Callaway. Like, 80% of the time. If he wasn't open, it was either a run by Grier or a sack. I didn't see him looking at his second or third option and maybe threw the ball out of bounds once. On the plays he didn't look at Callaway, he looked at only one other receiver and didn't look at several.
If MacElwain sat Callaway, then Grier would be FORCED to look for other receivers and not be so dependent on him..
The other option would be for MacElwain to give Grier orders not to throw it to Callaway.
Anyway, if Grier continues to do this, other teams will put their best defensive back on Callaway and cause problems.
Chances are, they'll get it corrected, but I still could see MacElwain doing this as a way to get him to look for other receivers.