There is. It's the result of Asmodeus's attempt to recover the "shard of evil" from the heart of the Abyss. (See MotP, p 89.)
This is one of the (many) cases where I can't really see the difference between NE and CE. What do bugbears, gnolls, ogres etc do? They ruthlessly pursue their own self-interest and power regardless of rules or social convention.
Yup, there it is. Sorry, never was a big fan of multi-planar adventure, so some details tends to get ignored while playing.
On another note, I foud this thread on 'loths lore for the Dawn War, I dont know if it differs that much from their natural lore:
''Most people are familiar with the Great Wheel/Planescape version of Yugoloth lore. Created by the night hags and bearnoloth's(/Asmodeus in 5e), mercenaries in the Blood War, and there's a theory that they started the Blood War and may be the progenitors of all of fiendkind.
However, I never see much talk about 4e's yugoloth lore, which makes sense since it was sort of spread between several Monster Manuals, the Demonomicon, and several issues of Dragon magazine - and it was obfuscated by the fact that they weren't often called out as a distinct group, having been rolled into demons.
In 4e, when Obox-Ob fell through the Abyss he created the Blood Rift (in older editions the Grand Abyss), which has the notable property that no Demon Lord or God can enter it. It is here that the first demons (all yugoloths with -demon instead of -loth or -daemon in their names), who were loyal to Tharizdun ended up settling down. Because their master is in chains, and because no greater powers can enter the Blood Rift, they are free to do as they please and have developed into traders, emissaries and (especially) demonic mercenaries selling their services to the highest bidder.
That's what's different in 4e. Thanks to Dragon 417, most of the old Great Wheel lore is also possibly true, though if you want to harmonize it with 4e's other sources, I'd do something like: Obyrinths send Seed of Evil from their dying universe to a brand new one > Tharizdun corrupted, creates abyss > Baernaloths spawn as first demons of the Abyss > Bearnoloths create yugoloth demons > First ultroloth creates Heart of Darkness and turns the Law and Chaos in the yugoloths into larvae > Asmodeus steals part of the Seed of Evil at the heart of the Abyss (possibly being convinced by Pazuzu or the baernaloths) > Asmodeus kills his patron god and the first baatezu devils are the angels that fell with Asmodeus and the Lawful larvae from the yugoloths > all other demons of the abyss are spawned from the Chaotic larvae of the yugoloths > the yugoloths pretend to still be demons and settle into the Blood Rift as mercenary fiends''