My Min Pin pup peas every time in excitement or being scared , why is this?

This morning my fiance let our ninja ( min pin ) out of his home and in a clam voice called him out ninja came out so my fiance goes to put the leash on him and ninja rolls over and squirts pea all over my fiance hands so he just took him out side to finish his Business . Why does he squirt so much and he also likes to do his Business anywhere in the house , we take him out all the time but does nothing just peas we can sit outside for a long time but as soon as we get in the house hell do his nasty Business what can i do