How far down did you actually go? You didn't specify anything; all you said was that you got ranked down.
If you went down a few ranks, that would probably be due to a few "unhearty" posts; if you went down over 100 points, well, I really don't know what to say to you...
Yes, very likely. Happens to everyone at least once, I believe. And sometimes the system 'cleans' itself out and the number changes.
Well...ranked down...well you didn't think others won't gain AP too right? They just...earned more than you I guessed, and pushed you down.
That's the most likely, I guess. Especially at lower ranks: There are more users, which means more competition. As you get more points and reach higher ranks, you may find yourself not losing your rank at all but moving up very slowly too.
all you said was that you got ranked down.
You have a point: Ranked as in the little badges you get with Middle Aged social classes, AP or your rank in the users list?