I expect there was a good reason, like that the voice actor was sick or something. I don't see that there's any reason to get ones panties in a wad over it. It's not censoring; more like there's a reasonable explanation which a lot of bronies can't see cuz getting into spazflail raeg is far easier. -.-
After all, it has happened in one episode. That's right; one episode. Before anyone can speak of censoring, surely it needs to happen more than once? If Derpy ends up being changed over the whole of the rest of the second season, then yeah, it's obviously something wrong. But if that does happen, then a civil, carefully written out email to Hasbro or a comment on the creators' dAs is going to be more likely to be taken notice of, than a hate filled cussing session.
Below is a link to the dA of one of the animators. He does his best to address this issue.
Kreoss on deviantART