Gawker just posted an article about the Derpy issue. It seems.... Hasbro was to blame. According to the article a Hasbro PR had this to say
"The 'My Little Pony Friendship is Magic' series has always been about acceptance and inclusion, and the series strives to convey that through the playful antics of a diverse cast of characters," Hasbro spokeswoman Nicole Angello told us over email. "Some viewers felt that aspects of the episode 'The Last Roundup' did not stay true to the core message of friendship which is the heart and soul of the series. Hasbro Studios decided to make slight audio alterations to this single episode."
You can read the full article here