However, other posters have given a more plausible example: you have server hosting with someone who isn't also a registrar, so for simplified management, you get your registrar to point your domain's nameserver at the server hosting company's nameservers, delegating it all to them... now you just need one customer portal to update all your hosting and DNS entries... and then you leave the hosting company (maybe the bill's too much), and you also forget to clean up the NS records, pointing them back to the real registrar's nameservers, so they're still pointing at this server hosting company you're no longer a customer of.
The hosting company isn't a registrar and is under no obligation to follow ICANN policies, so they can give the domain to some other customer of theirs without doing any verification. (I'm not sure they can do domain ownership validation, if they're not a registrar themselves)