Ok, update. I was being a dumbass and causing a burr to be drug up from the bottom of the bullet upon seating. I thought it was a burr on the brass, but after closer inspection, it was copper from the bullet jacket. I went ahead and dumped all of the powder out that I'd already weighed, and proceeded to chamfer the mouth of every case. Wallah! The problem went away.

Weird thing is, I didn't chamfer any of these cases last year when I loaded them up, and there was no problem. The Lapua brass actually looked as if it already had a chamfer on it, but apparently not enough.

So, just an update, problem solved.