On Close flights it is a very fragile balance. Most flights are overbooked, especially during the summer. They may only be over from 1 to 5 and generally they will get everyone on. The balancing act starts when you are close, and a flight with connecting passengers is a little late arriving. You know there could be more passengers coming, but you also know, they might not make it. You can not bump someone on the expectation that the passenger will make it from the other flight, but you have to get them on if they arrive prior to fifteen minutest before departure. SO, that is why gate agents sometimes have to come on the plane and ask for volunteers. Nine times out of ten the compensation is lower if you volunteer, than if you are involuntarily denied. Those of us that schedule so we can be bumped are willing to take the lower amount rather than force the airline to bump someone off.