I'm just gonna come out and say it. Volpe leaving is the reason the show tanked. His coworkers as director during season two were:
Bisignano (Fish Hooks, Uncle Grandpa, Mixels)
Infantino (Camp Lazlo, Flapjack, Regular Show)
Piluso (Camp Lazlo, The Mighty B!, Phineas and Ferb)
Varon (Billy & Mandy, Clarence).
As you can see, his fellow directors came from shows that were chiefly episodic and, while there may be underlying lore and consistency within the show, it was sparse enough to allow flexibility within the show's plot. There may have even been a few callbacks or themes in the shows they used to work for.
Volpe, on the other hand, directed episodes of Avatar: The Last Airbender (S1E4, 8, 11, 15, 18, S2E1, 4, 8, 10, 13, 16, 19, S3E1, 4, 7, 10, 13, 17, and 19) and Star Wars: The Clone Wars (S1E17, 22, S2E6, 9, 11, 18, 21, S3E5, 6, 12, and 14).
In other words, Volpe's track record is with shoes that have an established story to them, and has worked on them from their first episodes all the way to the end. He's used to stories that have definite key notes throughout the run of it, as opposed to only episodic storytelling.
It shows, considering how messed up and unfocused the third season is without him there to give keen advice.