Sorry if this has been already said.
And I thought about this and got to the same conclusion as Nihilus, you can see my post at HaloSpain were I wrote this same solution before even coming to this forum.
So don’t say I copied Nihilus or Nihilus copied me because we got to the same answer without even knowing about eachother’s existence.
Well, here it is, from The Fall of Reach: The Definitive Edition (there is no change with the 2001 edition)
>>Chapter 36
>>0616 hours
Right at the final line of the chapter (about 0620), Captain Keyes orders an Evac op for the Spartans on the ground while Master Chief is in the Station. But we don’t see any actions being taken.
>>Chapter 37
>>0637 hours
Tells about Master Chief until his arrive at the Autumn. When he gets abroad (let’s suppose at around 0650), Keyes appears much more frightened and considers Reach lost.
Nothing is said about that Evac operation.
So, the Pillar of Autumn had around 30-40 minutes to land, take Cortana’s part from Six, and try to rescue the S-II (though they don’t appear).
In fact, that explains what the yoink happens with that Evacuation. Even in the very first edition of the book, Keyes orders the evac operation but nothing more it’s said about it.
Nothing.
And has always been a 30-40 minute span. And that’d coincide with the time The Pillar of Autumn, the level of Halo: Reach, happens.
Halo Reach is canon. And so, this is the best answer I know of for the appearance of the Pillar.