I already explained it by going on about the Grimm being able to eventually 'get' a soul after they consume enough humans. Something along the lines of 'becoming' what they eat. That's why Grimm have animal-like forms, and why Faunus ALSO have many different animalistic features, which can't simply be explained through loldifferent tribes. We've literally have not seen ANY two faunus that had the same animal-features.
Essentially, 'ancient' Grimm evolved into Faunus, and eventually began to reproduce amongst themselves due to that. That's why faunus weren't involved in the ep 1 mythology creation myth. They gained their soul through the consumption of lots of humans, which is why they sought to eat humans the first place, as eating humans for food is stupid, seeing how Grimm don't have souls, and thus, aren't even 'allive' the first place.
Basically, an instinct to get something that they don't have.
So screw you. D:<