Cloudlands of Avaeraether
Supposedly the realm was destroyed in a titanic conflict with the dragons nesting in the stormhorn and other mountains surrounding the stonelands. The dragons also died, drastically reducing their numbers and it infers that the goblinoids in the area were able to flourish because of this.
"Most legends agree that there was once a powerful magical kingdom above the Stonelands, around -400 DR, kept secret from those races who dwelt below it on the ground. This kingdom, which supposedly existed 1500 years ago or more, boasted a society comprised of dragons, giants, sylphs, pegasi, asperii, giant eagles, and even some winged, elflike beings whose name is no longer remembered (and who are now apparently extinct). These races coexisted in peaceful harmony among the clouds, avoiding the decadent human realms of the time, especially Asram."
So giants from the destruction of Netheril move into the stormhorn/thunderpeak/desertsmouth mountain range region (which I reckon could have all been joined together).
Sometime in -350's the cloudlands come to the attention of Thauglor who launches an all out attack. The giants release an epic spell of some kind at the same time that sharn magic is altering the geography of Anauroch.
Cue some kind of magical resonance. The an entire section of the mountain range gets teleported or thrown into the air (shredding the dragons and giants en route) and lands in the stonelands.
This weakens Thauglor's forces and stops him from exterminating the elves in his realm.
The stonelands are now filled with stone, but still quite fertile (just a bit more rocky), and the hobgoblins are free to move into the mountain ranges (along with orcs and everything else).