Clearly OU is warming up on that side of the field. The TCU locker entrance is in the right-corner of that end zone, but instead of coming out of the locker and running up the sideline to the other half of the field for their warmups, TCU cuts across the back of the end zone and some players are actually IN the end zone. Obviously Mayfield was already throwing passes there, and in the bottom left of the screen you see a receiver out to catch a ball and he sort of walks away at the last second.

Now, I'm not mayfield fan. However, even in a high school game, if you were throwing warmups and a team ran in front of your intended receiver, I bet a many if not most QBs would still uncork a ball in that direction out of spite.