E la acumulacion geográfica de minerales
RE

when you rub, you also apply pressure, that pressure can urge the poop to come out. i think that when your rubbing, you also might be slightly breaking it up and that can further fill the stomach.  

age might have something to do with behavior depending on the age because not everyone who is the same age acts the same like 2 year olds could have bad behavior but some won't.

i hope this : )