Quinn would have left last year if the opportunity presented itself. He is likely eaten alive by his tenure in Atlanta and how that SB meltdown changed his career and the perceptions of him in the sports world. The man is an excellent coach, has proven himself in that regards and burns to be a winner as such.
I think he wanted to return to SEA and figured this was Carroll's last season but they were one of the surprise teams and Carroll doesn't show any signs of hanging it up. The guy seems to like coaching more than the other 31 HC's in the league.
So, he has to consider other opportunities. And he is at the peak, every GM looking knows Quinn's D played well enough to win that game.
And some assume he would want the HC gig in Dallas. Maybe after he's looked behind that curtain and saw Booger put McC through that press conference using Payton, he might be thinking this is not the place to HC.