USA3000 runs both charter and scheduled service. If one of their aircraft goes tech or they get aircraft in wierd places in the rotation due to delays or crew availability issues, particularly towards the end of the month, they can and will sometimes reach out to another carrier to fly one or more legs. It's called "subservice" (for substituted service), and it allows them to keep their pax happy by having an airplane at the gate at the appointed time. Subservice works best if they know in advance that they're going to have a gap, so they can survey other carriers and see who can fly the leg for them. This costs them money, but it's smart business in the long run. That you showed up at the gate and there was an airplane there at the right time, albeit on a different carrier than you expected, means that (assuming that this was subservice), someone ordered that aircraft most likely at least a day before. ATA, NAA, Miami Air, Ryan and some others can be pretty responsive when they have to be, but more notice yields a more reliable replacement. As a practical matter, military flying is a little slow this quarter, so ATA had an aircraft available.