All phenomena are caused by nature trying to correct one or more violations to conservation laws. A violation creates an imbalance which must be corrected. Newton understood this as the law of action and reaction. Acceleration or deceleration is nature's way to correct the imbalance. In the case of a moving clock, nature will slow it down or speed it up to correct an energy imbalance. I cannot go into what causes the imbalance here because this is neither the time or place for it. It would require a dissertation on the nature of motion.