I have a cat that keeps going outside his litter box (poop)? What could be the cause/s of this?

He has two litter boxes because I work long hours and away for two days sometimes. Also he is not neutered. It was suggested the behavior could be because he is not fixed, but a friend who has multiple cats who are fixed do the same thing out of their litter box sometimes. My cat does it more than twice a week.
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