What is the recipe for an upset...?

Nothing to lose attitude?...

PARITY. The NFL has strived over the past 15 years through player free agency and trade conditions to ensure every market (i.e. "team") has the ability to compete. This year it seems to have finally taken total effect. Any team on "any given Sunday" can win versus any other team.

Nothing wrong with it. It in fact increases the watchability of each game and definitely thereby lines the pockets of those who stand to gain from better competition throughout the league.

Overlooking an opponent based on their record to date

Coming off of, or playing next week, a game with a major opponent

Coming in with a much better record, but the individual match-ups at key positions are not in your favor

Poor play selection

Trap game (definition: a "lesser" opponent sandwiched between two bigger rivals)

Give up big plays

Turn the ball over

The key ingredient seems to be dumb, stupid mistakes being made by the team that is favored to win and the underdog taking advantage of those dumb, stupid mistakes....fumbles, INTs, penalties that hurt their teams forward progress. underestimating their opponent.