Small-sample size. In several ways. First, Dominion has a lot of luck in it. Seven games isn't enough to wash that out, so you'll always see some 'upsets' that are more about card distribution than anything else. Second (and more importantly), it's a small tournament. A couple big upsets (Me, RJ, Marin) will make things seem really wacky, but don't ultimately mean a whole lot. We are primed to look for interesting results, so we ascribe more significance to the upsets. Have there really been a bunch more upsets this year compared to last?
I bet once it's all done, the number of upsets won't seem ALL that far out of line with expected results.
Oh, and I bet that a fair number of lower-ranked players could be 5-10 levels better if they really focused. Tournament games inspire that focus.