A 6 year old may poop his pants for a number of reasons.
It's pretty normal in most cases for a 6 year old to poop his paints occasionally.
However if the 6 year old is pooping his pants all the time or everyday then he may have some bowel issues that should be looked at by a doctor.
But if the 6 year old is just pooping his pants every now and then it could be simply that he is just too busy playing and occupied with what he's doing that he forgets to to go to the toilet and or is trying to hold it in to avoid going to the toilet and then eventually loses the battle and poops his pants.
I remember pooping my pants as a 6 year old kid and my brother did as well.
I also have a 6 year old son that is almost 7 and sometimes he will try holding his poop and then eventually poop his pants because he just didn't want to stop what he was doing.
However if the 6 year old continues pooping his pants and it happens too often then the 6 year old pooping his pants may also have a condition known as encopresis.
Encopresis, sometimes called fecal incontinence or soiling, is the repeated passing of stool (usually involuntarily) into clothing.
Typically it happens when impacted stool collects in the colon and rectum: The colon becomes too full and liquid stool leaks around the retained stool, staining underwear.