The cause of a child to suddenly start pooping their pants can be caused by a variety of different things depending on their age.
If the child is just recently potty trained and you have another baby in the house who is in diapers then the child may be pooping his or her pants so they can get attention from you.
Or they might have a medical issue as well such as bowel incontinence that leads to the pooping of the pants or they may just be too busy playing and not want to go to the toilet.
My son who was 5 years old started pooping his pants when I had a new baby and he was regressing and since he didn't stop pooping his pants I put him back in some Pampers diapers that fit him.
I went out and bought the Pampers size 7 diapers for him and told him that until he stopped pooping his pants and wetting his pants he was gonna wear diapers all the time.
He was pooping and peeing his pants no matter where we were and there was nothing wrong with him.
He hated wearing the diapers but he wore diapers for a year after that until he turned 6 years old.
Because he continued to pee the diaper and poop the diaper even when I asked him if he'd rather use the toilet.
It took a year for him to grow out of the phase but at 6 he decided he would use the toilet and stop wearing the diapers.
I didn't mind having him back in diapers all the time at 5 and it wasn't that bad to change his diapers.
If your child is pooping their pants and refuses to use the toilet at all then put them back in diapers for awhile and they will eventually stop doing it.
It's much more sanitary for the child to poop or pee in a diaper instead of their pants and get it all over everything.