It's simple. The Bills are 7-5 going to an away game against the 9-3 Broncos. Manning clearly looks like he lost something and the Bills have a chance. Bills are down 7-0 and it's 3th and 1. Orton dekes a defender and slides two yards short of scrimmage line. There is a good chance he could have made it to first down marker. It's obvious Orton made a business decision. Bills lose by 7 and miss the playoffs by a game.
He did play well imo and the Bills didn't miss the playoffs simply because of that (losing to the Raiders was worse) but that play was so memorable.