Nothing's missing. Translocations are a common cause of infertility, but non-lethal ones can be inherited. All it takes is one breeding pair whose translocations happen to be incompatible with the genome of most members of their species, but compatible with each other's, and you'll start getting perfectly fertile offspring who are only fertile within the subgroup carrying these translocations (i.e. produce no offspring or sterile offspring if they mate with members of the original population). Presto! One species has become two.
And which conveys a competitive advantage, or at least isn't a disadvantage...or your new species will get eaten by a predator or otherwise outcompeted within a short while, and that's the end of the new species.