Why do cats poo on beds?

how come my stupid (but nice) cat, elvis, decided to take a nice crap on my duvet cover? I come home all excited to go to sleep when i smell this gross stench of cat poop. there may be many reasons for why he did this. probably because he wants to go outside and it was too late for him to go out or because he is too lazy to go to his litter box. or maybe the door was just locked and he couldn't get out. he could even be sick! hopefully this behaviour won't happen again.