Coming out of lurking to offer some insight (by which I mean, join the speculation). While I don't think it is common practice, I do know from an email exchange with the admissions office that there were some IT issues roughly around the time of the arctic blast that hit Chicago. Since that was so close to the original deadline, it is possible they extended the deadline for applicants who were negatively affected.
Another possibility is that the number of applications received was below internal projections. While this does suggest a less competitive applicant pool and maybe makes it easier to get in, the double edged sword would be that there could be something in the mpp market driving applicants away from Harris. Here's to hoping that's not the case.
Long story short, I wouldn't get concerned about the extension. The applicants who rightfully needed it will be able to take advantage, while those who were too disorganized to get it in by the proper deadline probably suffer disorganization in the application components (GRE prep, transcript, etc.) and probably aren't threats for grabbing a slice of that scholarship money we're all jockeying for.