The yearly format is that a rotation occurs every 1st September, but it was rotated early this year, and rotated 7 sets to get rid of the entire SP engine, Sableye and BTS. The only claimed reason was to lose Sableye in time for Worlds, though.
I don't know much of the pre hgss-on Meta other than dialga chomp was the best. I feel llike this is off topic so is there anywhere I could learn up on things like what sableye did and what the sp engine was?