It seems that when an author goes up for archive, if they have any of their stories written in collaboration with an author who has already had their works archived, those stories become premier-only immediately rather than after the week grace.
OldBoy101 has two collaborations each with two authors already-archived:
Tinus with story IDs 57297 'Abandoned and Rescued' & 54851 'A Taste For White Meat'
Penqwin with story IDs 55479 'Janine, The Little Sister' and 57435 'Kerry Had a Little Lamb, the Conclusion'
These four stories are now marked premier-only even though the weeks grace does not expire until the 10th.