It may have been because he was "disorganized".
Your point that Garrett should have weeded him out in an interview goes back to my point that Garrett didn't have many direct coaching contacts or working relationships in the coaching community before he came to Dallas. When you have no personal background with people, you have to resort to less than reliable routes like interviews and word of mouth. If I had a nickel for everytime someone I hired who interviewed great but didn't work out longterm, I would have like fifty leven nickels.
FWIW, this is reportedly one of the things that got Andy Reid in trouble in Philly. He worked with one defensive guy for so many years that when Jimmy Johnson passed away and had to be replaced he had lost all of his defensive coaching contacts from his assistant days and didn't know who to replace him with. He tried to promote a young guy from within, who wasn't ready for the job and didn't work out. Then he had to go outside and bring in guys based on reputation and word of mouth like Jim Washburn, and it was a train wreck.
Despite labeling myself a "realist", no one understands my pain or appreciates my special truth. Stupid world.