I haven't heard this from Heupel himself, but I bet he's got chips on his shoulder from before the OU firing as well. Peruse a quick bio on him or his Wikipedia page. He's been something of an underdog his whole life.
He was born and raised in Aberdeen, SD, which makes small East Tennessee towns look like they are located close to the big city. Good HS player, but it's South Dakota and he wasn't highly recruited. Wasn't offered by any FBS programs and goes to FCS Weber St in Utah. Blows his knee out, gets lost in the shuffle there. Then goes JUCO route, has to battle for (and wins) playing time, plays well, which gets him an offer from Oklahoma. Has a great career there, but isn't seen as an NFL prospect, ends up being a late round draft pick, and ultimately doesn't even make an NFL roster. Begins a college coaching career, starting at the lowest rung on the ladder (GA) and working up to be OC at his alma mater, but tenure there ends badly and he has to restart at another small school (Utah St).