There's a discussion over in Skyrim General that the current Dragonborn is a Shezarrine, commissioned by Lorkhan to keep Alduin at bay to preserve Lorkhan beloved Nirn.
So Shezzarines are all connected to each other (and to Lorkhan) in some way, that could reasonably explain why the current Dragonborn is what he is (divinely related to TS/Talos), who is summoned with the power to destroy dragons and absorb their souls.
But then, why was Tiber Septim a Dragonborn in the first place? Unless I'm wrong about the time line, there's no dragons during the time of Tiber Septim and his ascend to godhood Talos. If he was a Shezarrine; did Lorkhan just made him Dragonborn so that the Septim bloodline would be definitive and powerful from then on?
Also, why didn't Lorkhan just directly intervene (the way Akatosh did against Dagon) during the First Dragon War and just say "Not now, Alduin!". And also, why would we need a Dragonborn anyway? Any mortal, with training in the Voice and could hit hard enough can take down Alduin!
Thanks!