With travel costs on the rise, many desirable applicants are cancelling interviews. Hearing that you will be in the area, a school may take a look to see if you are on their list titled, "Applicants we should invite for interview". If you've made the list, they'll be glad to extend an invitation knowing that that you are likely to accept.
If you haven't made the list yet, then they will tell you that they can't make an offer at this time. As it gets closer to "closing time" you might look good and they'll contact you, even though it is later than you would have liked.
At that point, you'll need to decide if you want to expend the resources necessary to interview there or if you will decline the offer. (At which point the school kicks itself for not grabbing you when you were in the area.)