I've never had this happen until just now, but it seems that if you're making loops around an area, you'll begin to receive less and less exp/karma/money over time. I ran with a zerg in Straits of Devastation for about an hour before it reached the 80% mark, and shortly thereafter, the 95% mark.
I didn't actually notice this until someone else realized the same thing was occurring for them and mentioned it in area chat. Multiple people began posting their rewards after each event and it appears that there's several levels - 20%, 80%, and 95%.
I don't know how long the reduction lasts at this time, so if there's any information on this out there somewhere, it'd be nice to know.