I will suggest an alternate hypothesis - hunters suit the play style of the best players. Both bladedancer and gunslinger have very high agility by default, and can reach higher maximum agility than any other class. Also gunslingers have a couple of useful weapon-based perks on their class list. If you are a top-tier twitch-shooter, bringing Thorn+shotty or sniper, and you expect to not get hit, it is the clear choice. For me, I do better as a warlock or titan because I need the armour. I get hit a lot. I'd be very interested to know if the hunters are mostly gunslingers or bladedancers. I'd guess bladedancers - the blink jump, the crouch-triggered invisibility and the full-weasel super probably give it the edge.