What controls the bowel peristalsis is the brainstem. If you feel constantly stressed, the brainstem commands the sympathetic nerves to become dominant and therefore it makes the parasympathetic nerves become somewhat dormant.
Meanwhile, the vagus nerve, one of the cranial nerves that regulates the functions of the internal organs such as digestion and heart rhythm, belongs to the parasympathetic nervous system. Therefore, we can say that nervousness such as from anxiety, despair, guilt feelings, anger, etc. causes the vagus nerve to be inactive, thus bringing lower immunity, indigestion, slow bowel movement, constipation, irregular pulse, etc.
While it is still a theory, muscle stiffness also affects the vagus nerves. For instance, tight neck muscles may press adjacent vagus nerves and affects the functions of the internal organs. Tight muscles can also disturb neurotransmission of the enteric nervous system that regulates the digestive organs with or without the vagus nerve.
It has been empirically reported that sudden bowel movement occurs when certain muscles are massaged and their tightness is released. This would be one of the manifestations based on the above theory that suggests that releasing muscle stiffness could innervate the vagus nerves or the enteric nervous system, as all the nervous systems are connected through ganglia.