How do you envision some of the houses early on came to side with Nymeria/Mors? Maybe some were in the same position that Argilac was in but instead of fighting they allied to the cause to make sure their houses kept their lands as under Nymeria no one would stand in their daughters way to inherit. Surely they cant all have been beat into submission. Mybe some on the water ways also came for the technology and knowledge to make their lands more productive especially given the ”water witches”

Well, we can already make some educated guesses from the text. To begin with:

“the Rhoynar brought considerable wealth with them; their artisans, metalworkers, and stonemasons brought skills far in advance of those achieved by their Westerosi counterparts, and their armorers were soon producing swords and spears and suits of scale and plate no Westerosi smith could hope to match. Even more crucially, it is said the Rhoynish water witches knew secret spells that made dry streams flow again and deserts bloom.”

That’s a very attractive package for a Dornish lord or King, so I can see a good number of them signing up for material gain.

Equally important, the gender balance of the Rhoynish invasion/migration helped to encourage houses to join them – with eight out of ten Rhoynish being women, Nymeria’s policy of mass marriage incorporated hundreds of Dornish houses into the Rhoynar or vice versa, depending on how you look at it, with no one losing anything and the local Dornish getting wealthy brides. 

Moreover, from the text we can see some geographical splits between the Dornish houses. The Yronwoods, who were ruling half of Dorne, viewed the Rhoynar as a threat to their predominance, and were joined by their bannermen, the Jordaynes, Qorgyles, Blackmonts, and Wyls. 

If you look at a map, this coalition is concentrated in Northern/Western Dorne, but in the eastern and more northerly part of that region. By contrast, the Fowlers, Tolands, Daynes, and Ullers (who allied with Nymeria and the Martells) are mostly regional rivals who occupied the western and southern sections of the same region – these lords likely joined due to pre-existing tensions with the Yronwoods. 

Further evidence of this self-interest comes from the fact that, of the six kings who Nymeria sent to the wall, two of them – Vorian of House Dayne and Garrison of House Fowler – were former allies. When you consider that Benedict of House Blackmont and Albin of House Manwoody were also mountain kings who “claimed dominion over the Red Mountains,” one definitely gets the sense that the Martells’ control over the western borders of their kingdom was shaky at best for some time.