A war waged by the Prince of Destruction needs no other reason but to destroy....anything and everything. We cannot superimpose a need for strategic importance upon Dagon's decision to march across the land, for to do otherwise would be to forget that the Daedra do not think as we do.
Additionally, Dagon hates Morrowind, for the Dunmer, through its Tribunal Temple, branded him a "Bad Daedra" and made worshiping him punishable by death. And considering that out of the three Great Houses on Vvardenfell, Redoran was the Temple's biggest supporter and defender, one can see why Dagon chose to attack its capital first.
Yours in the Scrolls,
___The Word Merchant of Julianos
Have you taken into account the tale of The Greedy Man, the Leaper Demon King and The Dragon? Wether it is accurate it may still point to something underlying Dagon's actions in addition.
Lord Hyamentar
Bethesda story writers got a bit high and wrote that Hlaalu and Dres outlawed slavery. There was a civil war, the Temple split (somehow) and Ald'Ruhn got obliterated, Vehk flew the coop and the Nords invaded.
Happy days.
Working on the supposition that all these are rumours have you taken into account that some rumours are recent and some refer to past events? Also rumours can be even more innacurate than the most agressive propaganda. People seem to be assuming that all the rumours in Oblivion are about new events - but is it necessarily so? The gap from Morrowind was only 7 years, right?
For example we already know of a split in the Temple - the Dissident Priests led it. Now what if this rumour referred to that old news?
And there was a civil war on Morrowind that lasted how long - and then the Nerevarine turned up and sorted it ...
Then as proweler said the Nerevarine went North to Solsteim where that Nord Island was being settled by Imperials ... I wonder if the Nords then sent a force to Solstheim and wiped out that impudent Imperial settlement later?
And ofc the Nerevarine later went South to Almalexia - aka Mournhold and sorted that. I was looking over his shoulder then too
Mind you I didn't see you when you were looking over the Nrevarine's shoulder in Solstheim proweler.
What you might ask is what might rumours in TESV have to say about Oblivion and Morrowind rumours and events? That is assuming the timeline goes forward as it has done to date.