So I'm a GM that gets there on segments. Over 70 already this year. I pretty regularly get upgraded early on flights out of my home airport. I'm flying on the 14th this week and had upgrade for first clear yesterday morning.
I have no idea how the upgrade priority works with Delta as I'm never close on the list when I fly from anywhere else.
I am also a Platinum card holder for what it's worth.
I've always assumed that it had to do with the frequency that I am on this flight. Maybe some sort of little token.