On-body communication channels are of increasing interest as more and more wireless devices are wearable in medical, military and personal communication applications. Based on the previous study at 4.5GHz, this paper investigates reasons for time-varying fading of dynamic on-body channels. Antenna movements during action are categorized into intentional movement which can be controlled and unintentional movement which cannot be controlled. We use commercial software POSER7 to generate intentional movements between on-body antennae, which include distance and direction mismatch. The unintentional movements are considered to be a Gaussian process. Combining antenna movements with a measured on-body antenna radiation pattern, dynamic relative path gains during an action are obtained. Through statistic analysis, it is found that the middle and long term dynamic fading are mainly attributed to intentional movements, and the dominant short term dynamic fading is mainly attributed to the mismatch of antenna direction resulting from unintentional movements. New discovered long term error channels indicate more states should be considered in the Fritchman model.