There are many reasons for the regression. First, of course, is that the Eagles are more successful at bringing pressure while still managing to keep receivers covered in the secondary. Defensive pressure plus well-covered receivers means quarterbacks either have to make a poor throw, throw the ball away or succumb to a coverage sack—and none of these scenarios are ideal.
Flacco fell prey to all three of these situations this week, and he wasn't helped out much by offensive coordinator Cam Cameron's play calling either.
It was as though Cameron chose to address Flacco's problems passing the ball by asking him to pass even more, rather than increasing the carries for running back Ray Rice as a way to relieve some of the pressure on the quarterback.
Last week, against the Bengals, Flacco threw the ball 32 times, compared to 23 total runs (10 of those belonging to Rice; eight carries total went to Anthony Allen and Bernard Pierce once the Ravens were up 41-13 in the fourth quarter and there was no reason for starters to remain on the field).
Against the Eagles, however, Flacco threw 42 times while they went with the run 21 times, with 16 carries for Rice. This wouldn't have been a problem if the passing game were producing major dividends, but it wasn't—of those 42 passes, only 22 were completions.
Another reason why Flacco had success in Week 1 is that the offense heavily relied on the no-huddle to prevent the Bengals defense—particularly their defensive line—from substituting players, and the Ravens ended up with choice matchups as a result.
The Ravens slowly moved away from the no-huddle as the contest against Philadelphia progressed, choosing instead to protect Flacco from pressure by having him throw out of the shotgun.
However, all the shotgun passing in the world won't help if there aren't any receivers open. Nor will it help if the pressure comes anyway, forcing Flacco into making poor throws. Many times, Flacco overthrew his receivers—that is, when they weren't struggling to gain separation.
Flacco also threw the ball a lot on 3rd-and-short situations, including this 3rd-and-2 from his own 45 that resulted in an interception.
Every Ravens receiver was well-covered, but also singly so.
Instead, Flacco chose to throw to tight end Dennis Pitta in triple coverage—and, well, you know the rest.
Flacco cannot carry the Ravens offense by himself. Baltimore has one of the best running backs in the league in Rice, and a player like that should get more than just 16 carries when the quarterback is struggling. Asking Flacco to just keep passing made no sense—it's not as though the Ravens found themselves behind by a significant margin at any point in the game.
Further, the no-huddle was clearly a successful tactic last week; it was a mistake that the Ravens all but abandoned the approach against an even more dangerous defense like the Eagles'.
If receivers cannot get open, if the offensive coordinator refuses to call run plays, if the strategy that worked so well in the previous week is taken off the table and if 3rd-and-short becomes a pass-first down, well then of course Flacco, just like any quarterback, would have an off day.
Flacco's decision-making when asked to pass also didn't help matters much, but ultimately the reason for his down performance came from baffling play calling that failed to play to Baltimore's offensive strengths. If Flacco is to be a more consistent quarterback, then he needs more consistent play-calling out of his offensive coordinator.