ORIGINAL: SwampCollie
There is a better than average chance that your blades might be ever so slightly bent... that would change the flight characteristics I suspect. Heck depending on your broadhead the ferrel might even be a bit bent. Aren't too many of them out there that are extremely durable.
MOST will not even notice running into a foam target. The blades of a fixed head can't bend enough to affect the flight of the arrow as much as inconsistant form of even the best of archers. I'd say bow tune, arrow on the edge of spline or just got more tired and tired. I'd vote bad form and habits... or maybe even worn out arrows.
Of course, all this is based on assuming you're shooting fixed blades. Mechanicals bring in another possibility... premature opening which would definitely screw you and change flight.