Origin: StarCraft

The term "Zerg Rush" comes from the practice of manufacturing so many Zerg units in StarCraft that enemy forces are overwhelmed by the sheer numbers and force of the numerous advancing units, and it's a wildly popular tactic. As far as internet usage, it's common to see this phrase as different types of macros depicting a large group of people/items/etc ganging up on a singular person/item - like a million fanboys dog-piling on the last special edition of an action figure they've been waiting for.