Why does my cat always GO outside of his box?

My cat, chester, is about 10 years old now and for like 2 yrs he will poo on the bathroom rug EVERY TIME he is alittle upset about something. Usually its when i leave to a friends for the night which may be a separation anxiety issue but he'll sometimes do it if hes just plainly upset about something idk what hed be mad aobut hes treated like one of the family and gets ALOT of attention but i cant keep cleaning up after him. Oh also he stopped it for almost a year them we moved and it started about 4 mos after that...