Do you use some kind of whois privacy? If not, all your contact details are online, publicly visible anyone to who cares. If a whois command is installed on your system (Linux, macOS), open a terminal and type: whois example.com Replace example.com with your domain name. You should see contact information for the owner of the domain.

There are dubious services that scrape that info of newly registered domains and offer their services.

Edit: For some TLDs, whois privacy is not allowed. Depending on your TLD you may not be able to activate it / use a privacy service. You could always enter incorrect information but in a dispute you might lose your domain.