If you were a spectator during the crisis and the Septim line perished ( the supposed living descendants of Talos ).
And in the temple of one Akatosh sundered Dagon going with it the Amulet of kings the other tie to the even more ancient bloodline mixed possibly with both Shezzar and Akatosh.
The act of Martin becoming the Avatar of Akatosh is simply a form of apotheosis. Every true apotheosis occurs through a world-shaking event.
At roughly the same time an ancient enemy returns to seek vengance on the eight, only to be destroyed by the CoC acting on the behalf of the older form of worship..
Would you not feel that something had shifted, and that Talos had waxed and now was waning in descent?
The fact that a Shezzarine returns has nothing really to do with Talos directly. It's just that Shezzar is dealing with his old enemy in the same way he dealt with him the first time. Pelinal was the original Shezzarine after Shezzar first left the Nedic people, and they were conquered by the Ayleids. Pelinal-as-Shor helps Alessia and Morihaus defeat the Ayleids and create the first Empire, dies, and returns when a vestige of the enemy of the Nedic people returns. Dagon wasn't Shezzar's problem but Umaril was, therefore he returns to deal with it, and the CoC walks the path of Pelinal, so by association, he walks the path of Shezzar and becomes the Shezzarine.
It's the same story in every nation, as each nation has its own guardian. The Khajiiti have Baan Dar. The Redguard have The HoonDing. The Dunmer have the Nerevar. The Altmer have (had) Trinimac. The Nords have Shor. The Cyrodiils have Shezzar.
I'm not sure about Bosmer, Bretons, or Argonians.