Ask HN: Every time I register a new domain, I get spammed

Every time I register a new domain, I get spammed by companies wanting to develop my new website. The messages don't come from my domain name provider - they come from third party companies. How do they come by this data? Or am I being naive in supposing that my domain name provider is not selling this information on?

Edit: I can see that it probably comes from WHOIS data. But how do they get tipped off that the new domain exists in the first place?