I wonder if the name may be a subtle nod to "The Chronicles of Thomas Covenant", by Stephen R Donaldson. This idea is based on the similarity between the backstory of the Forerunners igniting the Halo rings to try to destroy an unstoppable foe (and themselves with it) and the similar backstory in the Chronicles of Thomas Covenant.
In these "Covenant" books, the ancient Lords of the Land were wise and powerful beyond the measure of the current lords (like Forerunner vs modern humans). Yet, in the end they were unable to stop their ultimate foe. Rather than be defeated, they enacted the Ritual of Desecration and basically scorched the Land in the hope of killing both their foe and themselves in the process. Before doing so, they sent the people they could out of the land and set up 7 Wards to reteach people when they eventually Reclaimed the Land, so that their knowledge wouldn't be lost. There are other similarities in the backstory and the fact that in each case the foe still exists and now has to be defeated for good in the present time.
My thought is that someone at Bungie may have been a "Covenant" fan. They loved the idea of having an enemy so unstoppable that even an ancient society that seemed immensely powerful had chosen to destroy everything they loved, themselves included, just to give life a chance later on.
If that was the case, then the name "Pillar of Autumn" could be a nod to the "Colossus of the Fall" in the "Covenant" books. In the books, the Colossus was a "high, monolithic fist of stone", "an obsidian column" that was empowered to defend the Land against its enemies. An obsidian column is pretty much a pillar. If you change out the Fall for Autumn, then "Colossus of the Fall" could become "Pillar of Autumn" as a nod to the "Covenant" books for inspiring, in part, the Forerunner/Flood backstory.