It's useful to imagine this way: When a motor is forced to spin by some external influence, drawing more current out of it on the other end means it has to spin with more torque in order to maintain the same voltage. Here, the generators are connected together to supply power in unison. They must all be in phase with each-other otherwise the out of phase generator will find itself fighting with the other generators, and as beautifully demonstrated here will very violently lose the fight. You could describe this as a very cruel demonstration of destructive interference!