Midwest has always been particularly heave on the morning bank of flights.
My guess is that, with all the low fares and competition, and combined with the economic slowdown, there might be a higher percentage of leisure passengers than there used to be. Of course, leisure passengers take a lot long to go through security. They typically have children, stollers, etc., and many of these individuals do not fly nearly as often as biz travelers. This is just a guess of course.
I looked at the link you provided, and there are a few things that jump out at me. Midwest says there are "more people than ever" using the D concourse because of YX/F9's expanded service. I'm notr sure if that's true. I would think that the traffic is comparable to the height of the old Midwest when the MD-80s were in use. Perhaps someone has those numbers handy. I also think the biggest reason the D concourse checkpoint is backing up is due to the arrival of Southwest.
The linked page also mentioned Milwaukee as a "vacation destination". Interesting...I thought people only go to Milwaukee if they "have to."