Wait, wait, wait. I don't deny that the Dunmer likely burn their dead, but don't the Ashlander burial caves have mummified corpses sitting around? I'm pretty sure there were. So, while the modern, House Dunmer may cremate their dead, I'm pretty sure the Ashlanders did (or possibly still do) mummify theirs.
They surre do! The Urshilaku and the Candidates in the Cave of the Nerevarine candidates would be proud of you.
And that suggests that Either the Great Houses or the Tribunal have monkeyed around with the most sacred rites and rights! or ...
Well we know that the Tribunal altered things to create the Ghost Fence as you guys mentioned earlier - but is there more to it?
The Exiles from Somerset Isle followed their own ancestral custom - I suspect that was part of their reason for leaving to discover their own land - because they would not and could not mix and match customs with the Altmer, then or later the Daedra seem to have moved in - especially the Lady of Dawn and Dusk - and clearly she followed Urshilaku custom ... where the Nerevarine was involved ... but all the Chimer (later named Dunmer) started out as nomadic Velothi like the Ashlanders before the Great Houses began to form. Ashlander is just a term attached to those who remained more or less true to ancient custom and were given thename because they were forced to live in the Ashlands in the shadow of the Volcano.
So was the burning of the bodies to do with preventing unhappy spirits from manifesting?
It is likely that many were disadvantaged by the rise of the Houses - not only the Ashlanders, but also the members of smaller Houses that were absorbed into the large and many members of the Great Houses themselves - both victimisers and victims. Great power is not a comfortable thing.
Now if Dagoth Ur had been cremated would he have been able to return as he did?
The Tombs of the Saints and various Houses small and great for example - maybe that was what happened to the Great before Necrom was founded ...