You could try to make some argument about hives reaching certain sizes having batches of royal huggers (if we're going that route), and any that aren't distributed far enough to found a new or secondary hive are rendered neuter in vitro by the presence of a mature female, at which point it's more about the hive finding a use for them than them being a desired caste.
Could be anything from guard/bullet sponge to something more like an outcast/lone wolf on the outskirts of infested territory.
I don't think the handwavium is even totally necessary though.
