I had a female villager who I worked up to Master Builder status. After that I switched her to Farming to hopefully get her to Master level in that as well. I recently switched her back to Building and she has dropped down to Adept. How is this possible??? All that work for nothing and now she's old!!

yes, with events

Hmm, I didn't have any events, except learning to make stew... strange.

that's the only thing that I know of...

Did you have the gong completed at the time? If so, did you ring it?

No, I haven't gotten that far yet.

We've seen some reports in the forums about villagers losing some skill, and LDW is aware of it and looking into to it. But as the others have said there are events (gong banging included) that will cause villagers to loose skill.