Yeah, I know, this topic is already well covered but... Considering that we're seeing a jungle spreading across Cyrodiil (West Weald) this year, I thought it would be cool to speculate before the release of Gold Road!
Well, as you well know, something similar to a cyrodiil jungle appears during Sanctum Ophidia.
I remember that this brought up many theories for the entire history of Jungle Cyrodiil, which left me fascinated. I don't remember exactly the story of Sanctum Ophidia, but according to this reddit:
"Part of the Celestial Serpent's plan is to create a dragon break in Craglorn in order to return Tamriel to the Dawn Era and restart civilization." ///
In other words, Cyrodiil's jungle is simply what happens to Tamriel during dragon breaks/the Dawn Era, when the land undergoes a Wild-Hunt-like transformation.
And now, with Ithelia's return, could Tamriel be revisiting past memories, and that's why the jungle is advancing towards Cyrodiil?
(Look at the name of the new Forest... DAWNWOOD.... Dawn Era.... I don't know but... well xD)
I don't know, I thought about it in the shower and it seemed too intriguing to ignore LOL