July 2008 is right about the time Age of Conan launched. I wonder if that's really what caused the drop?
WoW is just the biggest player by far. Currently I host approximately 88% WoW guilds. The other "bigger players" are EQ, EQ2, Warhammer, AOC, FFXI, and LOTRO, with Warhammer being the biggest non-WoW game at 2%.
WoW just has so many more players.