When Welch came in on 4th down, I think Dino wanted to run a QB draw. He called that play all last year and this year too. I just think Clayton knew he was going to get stuffed if he ran, that's why he suddenly threw that off balance pass.
I don't think Dino wants to tell us the play call, because of either criticism or he doesn't want other coaches to know as they'll look for it in the future. Of course, opposing coaches know this, but Dino has that coaches paranoia.
I also believe the injury and frustration had to do with the pulling as well, probably a mix of the 3.