1. The rankings are relative so if everyone else or many others go up, but you stay the same you will go down relatively speaking. 2. My understanding is that REP between blockchain runs is an estimate, so there might have been an initial overestimate that was updated and replaced with an actual score. 3. One or more people could have down voted a comment of yours. Their reasons are their own and there is no way to know specifically why. TLDR Don't sweat it. If your REP is positive you are probably doing the right thing and it will work out in the end.