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.