After The Scarab Gong was rung and the ancient Gates of Ahn'Qiraj opened, the horrors of the silithid and their qiraji masters were revealed. Massive qiraji resonating crystals broke through the earthen crust of Kalimdor and the colossi of the hives in Silithus emerged from their cocoons beneath the hives. They heard the call to battle and rose to serve their masters. They cast their massive shadow upon the whole of Silithus and marched into war.
In World of Warcraft: Cataclysm, it seems that lesser hives (or rather, at least one) may also spawn colossi.