Your mouse always poops on you whenever you hold him possibly because he is actually scared of you or it is usually a coincidence. Mice do not have the same type of rectum as compared to humans which can hold the poop and choose where to poop. Visit a vet for more professional information.
Have you seen the animated movie G Force? It's about animals that are spies. One of the mice gets caught and he asked his buddies what he should do. They all yell "poop in his hands, poop In his hands!". So maybe your mouse is a spy.