I'm not sure if this question has been tossed about before, but is there some relation between the Night Hags and the Yugoloths deciding to leave the Gray Waste? (Well, insofar as the Yugoloths making it their "home" plane of operations.)
Was there a deal struck? A terrible secret the Hags unearthed to scare off the fiends? Have the Baernoloths made a pact with them? Since the Hags command the larva, did they shoo off the Yugoloths as part of some agreement there...? Is it a willing one or no?
Canonically, the Loths have relied on the Night Hags to transform them into Altraloths. Perhaps greater control of Hades was just one of the terms of that deal. It's also noteworthy that, while individual hags have their feuds, the whole race rarely unites against one particular group. So even if some of the Hags did hate the loths, most would be perfectly willing to deal with them. Also, the Gray Ladies aren't numerous or individually powerful enough to hold a plane against a determined attacker. Perhaps they rely on the Yugoloths for protection?
Well, there don't have to be any relation at all. As someone once wrote: "Indiferrence is the most common form of cohabitation."
I think it would be ninety-six different flavors of pure awesomantium if the Loths left the Gray Wastes because one of the Gray Sisters knew a secret about them so dark that the General of Gehenna himself gave the order.
Not sure if that would fit into canon, of course. But the idea that the sisters knew something that would scare the Yugoloths off a plane? That has a lot of potential, I think.
It also helps solidify their relationship as secret-keepers. The hags are the ones who modify the Yugoloth; the hags may know more about the Yugoloth and their dark than the Yugoloth even know.
Maybe whomever is the lead hag is his wife or something. 
Do the night hags work mysteriously like the hags in, let's say Macbeth, who just show up from time to time to manipulate the events, or are they more like a secret society or even a state?
edit: I see, there's already a whole forum dedicaded to them, so I go looking there first...
BTW I've discovered that this crazy frogger I am using as avatar is a kind of yugoloth called hydroloth. 
Was there a deal struck? A terrible secret the Hags unearthed to scare off the fiends? Have the Baernoloths made a pact with them? Since the Hags command the larva, did they shoo off the Yugoloths as part of some agreement there...? Is it a willing one or no?
I don't think the relationship between the Night Hags and the Yugoloths has ever been thoroughly explained. (Wouldn't they be, at least to some extent, competitors?)
That said, my personal take on it is that:
1) The loths never really 'left' the Grey Waste; they merely expanded into Gehenna. No self-interested being, especially one as self-interested as a yugoloth, would willingly abandon territory that it controls.
2) As much as they are masters of manipulation, the loths don't really have as much control over the Blood War as they like to claim (and think) they do. Having the Blood War raging in their own backyard wasn't really their idea, and moving most of their presence into Gehenna simply gives them more breathing room.
3) The yugoloths will never really "abandon" any plane that has the Wasting Tower in it. If any one thing is an icon of yugoloth power on the planes, it's Khin-Oin. It's also the yugoloth capital (to a greater extent than the Crawling City, the tower in Chamada, or any other site).
The relationship between the loths and the hags should be better explained in general. Any real warfare between them? Is there now an uneasy truce? Why aren't they fighting each other tooth and nail, both for Lower Planar real estate and access to souls? Or maybe they DO fight each other often -- but due to the divisive nature of both yugoloths and hags, it's not species-based like baatezu vs tanar'ri, but individuals and cabals of both races allying and plotting against each other incessantly?
Would either race finally decide they'd be better off without the other? Or does the Blood War keep the yugoloths too busy to deal with the hags (or vice versa, with whatever the hags are up to)?
Also, I thought the idea with otherwise unclaimed larvae/souls was "first come, first serve" -- whoever spots it first, gets it.
BoGr Guide to Missile Combat:
1) Equip a bow or crossbow.
2) Roll a natural 1 on d20.
3) ?????
4) Profit!