Does anyone by chance know? Can't seem to find this anywhere and this question popped into my head.
Presumably (I'm not sure, this is an educated guess) they called it an antminer because they have very many of them, all set to do a single thing and doing it well. One could compare their mining pool to an ant hive, where each mining ASIC is an individual ant, foraging for food.
This is a very common metaphor, because ants are fascinating creatures, whose actions are completely governed by very simple rules, which cause truly incredible emergent behavior. See: Ant Colony Optimization for a good example of this.