The averages are a bit tricky, since he rounds up and down, but the totals at the end are spot on.
Anyways, it's all about expectations. Luck, RG3 and Wilson came into the league shortly before EJ and changed our expectations of a rookie QB. The expectation for EJ was to be better than Fitz, and to take the Bills to the playoffs. Neither happened within his first 14 games, so he's labeled a bust.
He's still on the team, he still has time to get better, so maybe he won't be a bust in the future, but for now, he's a bust, simply because he hasn't lived up to (possibly unrealistic) expectations.
Being a bust or not isn't just the player's performance, but also the expectations of the player. Two players can have the same performance, but if one player was expected to be a pro-bowler, and the other was expected to be a bench warmer, one will be considered a bust, and the other a surprise.