As I understand, the Red Mountain erupted, threw the land into chaos, and Argonia took advantage and invaded the island. Many Dunmer left for Skyrim, others went to Solsthiem.

But why did Argonia attack Morrowind?

Centuries of slavery. Either that or because the Hist said so.

They also saw that Morrowind was weak and vulnerable for a variety of reasons:

• All members of the tribunal had been killed or disappeared
  
• The Eruption of Red Mountain already devastated the majority of Vvardenfell
  
• By the fourth era, the empire barely maintained a presence in Morrowind

So the Dunmer treat the Argonians as slaves while they complain about the Nords treating them like dirt? As long as they ain't enslaved!

Guess even elves can be huge hypocrites too