In my experience, most of the time, this is either behavioral or it's urinary tract condition (cystitis, UTI, etc.). Your vet can do an exam and urinalysis.
If it's behavior related, it could be you don't have enough resources for the number of cats.
This problem can sneak up on you because cat your will seem fine with the other cats, until one day and the cat has had enough and starts peeing everywhere. One solution for that is to add more litter boxes and see if the problem goes away.
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