I don't think your domain name was transferred as that is a lengthy process to get it back called a UDRP (Uniform Domain-Name Dispute-Resolution Policy). What may have happened is they were able to change the DNS, or move it to another host, etc.
I have heard of "hacking" being as simple as someone calling a hosting company and pretending to be the owner of a domain. Often times they do not even ask security questions. Make sure your are secure at your host and they are not lax with security.
I often get pissed when I call a company and the let me start asking questions about my account without any verification. Many times also, it is as simple as your email address or bithday, um, which anyone can get from social media.
Check it out and make sure you are secure at your host and they verify correctly!