For all it's worth, just looking over some routes off Flight Aware, they will occasionally on PHL-MSP use the northern route that bypasses Chicago/ZAU airspace.
NWA395 this morning ....
This route via DJB.FNT.J106.GRB bypasses ZAU, although it doesn't seem to be used that often. The AWI flight right behind NWA395 was routing the usual way through ZAU via DJB.J34.CRL.PMM.J70.BAE. Interestingly the NWA flights inbound to MSP this morning out of IAD, BWI and DCA were on the northern route bypassing ZAU via DJB.FNT.J106.GRB as well. Maybe ZAU is restricting traffic through their airspace at certain times of the day, or it's just a NWA thing ?