/dons his almost-a-moose hat

This is one of those things we're still trying to suss out, so the fact you're talking about it as a point of confusion is not surprising.

What we know is that the explosion of the agrius released the seal on Silvertear's aether and this either:
- Made it possible to summon primals; OR
- Made it easier to summon primals.

I'm inclined to think the latter, though it is not really clear which is true. Primals exist on other continents and have also appeared in Eorzea before, though whether this was outright impossible before the battle of silvertear skies or not (and by consequence, when it was made not possible) is up for debate.

Questions we do not have an answer to:
- Why was there a seal on Silvertear's Aether?
- Why was Midgardsormr prepared to protect the area at the expense of his own life? (there are mythological reasons, but myth is often greatly warped from fact)
- What is the proper significance of the Crystal Tower?
- What primals existed on the continent of Othard and how were they themselves summoned?

If you have ideas, by all means share them. We will often find ourselves off-the-mark, but that only helps the process of elimination.