Welcome to the modern airline boarding dilemma. Most airlines at some point in their history have tried boarding planes from rear to front which arguably should be the most efficient way to go, but alas human nature makes it so that the early boarders just take the first overheads they see as they go back thus by mid plane, all the front people have no place to put their bag and end up shuffling back then forward to board and reverse when arrival. It seams about every generation the airline will try for a hot minute to increase boarding times but quickly run into this same problem, despite markings, signage etc. Thus the airlines all have zones which specifically feed people (after considering revenue) onto the plane based on ability to store bags importance.