It is possible that they lost their lease on their existing store and have to move. Perhaps another business that is willing to pay more money is moving in. Or maybe O'Reilly is getting a better deal on the lease at the smaller store and is trying to save a little money on rent.
No one ever knows why stores move. There was a Kinko's in my city that was in a brand new building, however a few years ago they packed up and moved across the street to an old building. The current store is smaller and looks quite rundown from the outside. Apparently, Fedex wanted to save money on rent and moved into a smaller place.

