The first two days of the NCAA tournament provided a number of upsets. Two 15-seeds, one 13-seed, two 12-seeds and two 11-seeds all won, along with one 16-seed who may have won without the hindrance of the refs. Most commentators seem to be describing the upsets as being due to more general parity. That parity has been evident in the last few years with Butler's multiple trips to the Final Four, along with VCU's and George Mason's runs as well. However, an additional factor likely on display this year was the overall quality of the lower seeded teams. In particular, the usual bottom feeders that inhabit the 15 and 16 seed lines looked much better than usual. I think part of the reason was that many of these weaker conferences put their best team forward. In past years, many of these leagues would see their top team get knocked off in the conference tournament. The conferences ranked 20th, 21st and 23rd – 27th by RPI all sent their regular season champion. Of the higher ranked conferences only the Sun Belt sent a clearly inferior team that belonged on the 16-seed line, Western Kentucky. A couple of them sent teams that weren't their strongest, Detroit from the Horizon and Loyola from the MAAC, but those conferences sent teams that were at least in their top four teams. Add it all up and there was a logjam of teams that typically would have been 13 and 14 seeds getting pushed down to the 15-seed line. If you have enough good teams, some of them will pull off the upset.
Of course this might not be a sufficient excuse for Mizzou, as the one 15-seed that seemed like a typical 15-seed was Norfolk State.