I think the Reachmen have a certain affinity for Hagravens and not necessarily the other way around. The Glenmoril Witches, who are a coven who is know to have connections to Hircine, I think in this particular coven just happened to also be Hagravens. But thats just speculation.
I would think the Hagraven transformation has some connection to Hircine. It's obviously not Lycanthropy per se, but I don't see why it couldn't be a related condition, especially since the Glenmoril Witches are both hagravens and are devoted to that particular daedra. Further, while ravens are not particularly connected to Hircine (being neither predators nor prey, but a fairly intelligent social scavenger species) none of the other daedra seem to have a connection to beasts in any general sense.