This is the second round of commercial resupply contracts awarded. The first round awarded commercial contracts to SpaceX and Orbital ATK. The fact that both of the current contract holders suffered failures (read: they exploded) played a role in the selection of a third candidate as well. Though specifics of the selection process will be released later, NASA obviously wants to make sure that they have plenty of different routes to get supplies to the space station. These new contracts will last thorough 2024.