When I learned about the Civil War in Skyrim, I learned, that the imperials and Stormcloaks are in war, I also found out about the Thalmor, who started the great War in Cyrodiil and forced the Empire to sign a peace treaty about banning the worship of Talos since the Empire lost. Why did the Thalmor suggest that the worship of Talos should be banned?
According to the TES Wiki, "Thalmor (Skyrim)" article:
As a branch of the Aldmeri Dominion, the Thalmor seeks to unify the Provinces of Tamriel through one-government leadership, with Elves holding all places of power. The reasons behind the ban on the worship of Talos are shrouded in doubt, however, the two most probable hypotheses are either that there simply is animosity towards what Talos represents (human turned god and the victories of Tiber Septim) or that it may just be intended to cause distress in provinces like Skyrim and thus weaken the Empire. Deeply offended by Tiber Septim's victory in Valenwood, members of the Thalmor have sworn to root out Talos worship in the Empire.
Also, found a good post on Reddit - /r/teslore "Why did the Thalmor ban the worship of Talos?" by user, ginja_ninja describing it. You can read the full discussion, but this the most in depth explanation I could find (paraphrased somewhat):
The Thalmor, and arguably all Altmer, hate Talos first and foremost because of Tiber Septim and what he did to their homeland. In the dying light of the Second Era, the newly formed Third Empire had reached a tenuous peace with the Aldmeri Dominion because Septim and Arctus knew they didn't have the manpower or tech to conquer the Isles in the way they'd taken Hammerfell. Though he'd been prophecied to be the first to rule all of Tamriel, Septim was originally content to resign himself to owning the provinces of men and coexisting with the Altmer.
Then Vivec gave him access to the Numidium. And everything changed.
With the power of Walk-Brass and the souls of Wulfharth-Arctus mingled with his own fueling it, nothing could stand against the might of Septim anon Talos Stormcrown, a fabrication made into reality and registered unto the firmament by a stolen god. Not even the oldest and mightiest civilization on the planet, previously so content and secure in their objective superiority, could do anything but despair as it crossed their threshold and imposed ultimate destruction, demanded absolute capitulation.
And so in less than a day the ancient, proud, and noble denizens of Alinor were forced to bow before their conqueror, a human, with the only alternative being rendered into a roiling time-wound in the sea.
Over the next century, they were forced to watch this man they could only consider a war criminal venerated and sanctified all the way to the status of a god, elevated to the station of Lord of the Divines as a reward for his thievery and warping of the cosmic order. They watched generations of men live, die, and forget as the story became more and more twisted and the atrocities remembered less and less until this conqueror was the greatest hero of all mankind, absolved of any wrongdoing by the ignorance afforded by generational legacy and selective memory endemic to the very nature of man.
Now remember how long Altmer live for. It's been a little over six hundred years since the beginning of the Third Era. There are plenty of Altmer alive in 4E201 who were around to experience Tiber Septim's conquest firsthand. He isn't some ancient figure of legend to them. Hell, he's younger than a good number of them. Are you beginning to grasp how infuriating it must be to watch generations of humans live and die and spawn and pass on selective knowledge to their spawn that conveniently leaves out everything bad Talos did to achieve the greater good? To watch a bunch of foolish, barely-literate Nords beat their chests in vehement defense of a megalomaniac they only remember from the romanticized stories of their almost-certainly-illiterate grandfather's grandfather's great-grandfather?
The metaphysical justifications come second, and are a compound effect, abstracted by Aldmeri philosophers and theologians to further strengthen the oppositional rhetoric. First and foremost though, they want revenge. They want to right the effects caused from when they were wronged. They want to stop the momentum of those effects reverberating forward through a genealogy. But it becomes a complex issue since the enemy they're fighting is a progenic proxy. Their true enemies are all dead of old age. They're fighting a legacy. The only enemies they have now are arguably innocent, unable to truly understand due to their short lifespans.