Possible explanations I can think of:
1) People using up RUCs before they expire
2) Diamonds are booking the same fare class as you, earlier
3) You are booking lower fare classes than you used to (or others are booking higher fare classes)
4) DL is giving away selling more seats through FCM (I had a $112 offer on SEA-JFK-xxx), so the three people that were in front of you that usually cleared in advance, no longer clear in advance
For what its worth, I was thinking the opposite last week in that I was quite high on the upgrade list as a GM on a cheap ticket. I think I was 20 out of 40 for SLC-SEA. Usually, I would be down around 30 of 40 with my fare class.