Chronology of Eras

[Delsus]

I don't know of any evidence supporting the idea that all of the Umbral Era catastrophes were caused by beastmen primals. The events could have entirely unrelated causes.

We also know for a fact that Midgarsormr had kept the primals sealed during its reign, and the primals were only set free when it died fighting Garlemald. This happened only a few years before the game took place.

I think Midgarsormr only sealed the beastmen primals, the elder primals (Odin and Bahamut) who it seems roam don't seem to have been sealed, because Bahamut was sealed in Dalamud by the Allagan Empire, therefore he would have been somewhere (possibly roaming) and Bahamut did something, that caused them to decide to seal him away. This has to have been before the 4th Umbral Era (when Allag was destroyed) it coulld have been that Bahamut all but destroyed the empire, so the survivors could have sealed him.

I don't think there are records going back beyond the 6th astral era in Eorzea, so we may never know..... Lore Master Fern, I summon thee for clarification!!!