The version that Mayhem told us in Avernus was that he tried to ally with the Zuma's in order to fight our common enemy rouge monsters. When finally he tried to draft alliance with them, he said that Zuma should be lesser member of the alliance (they should answer to us aka puppet). That insult worked well enough to get another realm to declare war on already diplomatically isolated Avernus.
There might been also some other factors, like Mayhem not listening Zuma ramble about monsters 1 million CS strong roaming the wild west.
After peace with Morek, Arnor, D'Hara and Astrum Avernus dropped it's idea to migrate to west and took Nifelheim as "loan" from Arnor (you can see the terms of the "loan" as it's written down as a treaty). Newer Storm/Sun Kings haven't seen reason to really fix the thing, as the distance between Avernus and Zuma homeland is too big and not even sure if you can contact their leader right now.
This is the version that I remember at least you can ask original people from Avernus to confirm this, as there should be few left.
While great way to make story into a fairy tale Ketchum, I believe that 1 long is too much as it happened over 1 year ago( or about 10 in game, as my char is 31 and when it started he was in early 20ties) to get prestigious second long one story must be at least be 100 years old in game ofc.
Avernus during the time of Mayhem did not have any plans move to north, but when the small war started, Mayhem promised like us duchies from which to make new realms. Current Avernus position happened after successful peace talks between Avernus and Morek's allies.
