Thanks for the suggestions so far.
I don't think this is a development issue, but for your interest, my agitation is continuous, by the typical shuffle method. I rotate the stack of negatives every third cycle though the stack. The silver density seems excellent and uniform throughout the negatives, but the swirls exist on all the negatives developed at that time.
The anti-halation layer is an interesting idea, but I have no idea how to test that theory or why it should happen now. Again, I have been using the same film, water, chemistry and technique for upwards of 1,000 negatives without this occurence (plenty of other mistakes along the way, though).
Keep the ideas coming ...
Henry