Unite Dunmer brothers and sisters! One day revenge will be ours!
What was the motive behind this assault on Morrowind?
Centuries of the dunmer invading and enslaving the argonians. It was the argonian's chance to strike back at the race that had been persecuting and enslaving them since the Chimer/dunmer knew about argonians.
If anything, the dunmer had it coming and the argonians were only too happy to carry it out.
But surely the Dunmer would be used to the ash. They live on an active volcano!
The ones in Vvardenfell did. If you remember Tribunal, when you unleash an ash storm on Mournhold, everyone is extremely surprised that there's an ash storm.
Anywho, the situation played out like this:
MoT fell, and triggered Red Mountain to blow its top. To gain some perspective of how large Red Mountain is, I calculated it to be roughly half the height and length of Olympus Mons on Mars in real life. Now, because Red Mountain blew its load, causing the Inner Sea to continue boiling for 40 years after the explosions and that Vivec is still covered in a few feet of ash, one can deduce this explosions was FREAKING HUGE! There were also likely tsunamis that would have hit the southern and eastern mainland hard, on top of a ton of ash. Now, while the dunmer of Vvardenfell may be used to ash storms, the amount of ash that probably bellowed may have been more than enormous and the mainlanders are not really exposed to ash storms. To further add, Indoril had been in steep decline since Tiber Septim took over Morrowind, Redoran likely lost most of their leaders and land from the eruption, nord raids, and the Oblivion Crisis. Also, the Telvanni are not all that powerful, as the Oblivion Crisis gave them lots of trouble, and even they began to give way.
With all of that hell combined, it was the perfect storm for the argonians, who kicked MD's forces REALLY hard during the crisis to begin with, to storm Morrowind and have their revenge for 3000 years of persecution, enslavement, and invasion.