How did someone knows that the two different domain was owned by one person?

newbies here...
I have 2 domain hosted in hostgator, 1 domain bought from hostgator, and the other from namecheap, both domain has Domain Privacy Protection. Someone posted in the Forum of Domain 1 and tell something about my Domain 2. I'm wondering how did he/she knows that the domain 1 and domain 2 are owned by the same person? thanks