[tec27]

While this is a fairly good explanation of some interesting things about Starcraft, I think something truly interesting about mutalisk stacking in particular is its actual discovery date. It wasn't, in fact, discovered in the 'early 2000s', but rather in mid 2006, a good 7 years after the game was released.

Prior to this trick's discovery, players stacked mutalisks by right clicking the group onto a mineral patch or gas geyser, which causes them to clump up more than right-clicking onto the ground (July, who you mention, actually won at least the first 2 OSLs without the benefit of this trick). I would imagine that air units in Starcraft have a similar hack applied to their moving collision detection as the workers do with mining, as the exhibit a lot of the same behaviors (okay with being on top of each other while moving, spread out as soon as they stop moving).