From the blog announcement, it associates your account with all eligible accounts (share the same OpenId) as well as:
1.Grant you a +100 reputation bonus on the current site, as long as you have at least +200 reputation on any site in our network. So you’re not treated like a newbie on every new site we launch.
2.Copies your profile from the network site where you have the highest reputation, to the current site.
3.Locate any of your questions that were migrated to this site and make you own them again. It was impossible for you to own them prior to this point, because you didn’t even have an account on the current site!
When it comes to multiple accounts on the same OpenId that aren't all associated, it targets your highest reputation first (probably to perform point 2 and also check for point 2), and grabs all accounts that are associated with that specific account. This is what I experienced when I created my Gaming account.
Haha, wow, that's crazy. It seems to progress in order of site creation. I checked myself, as well as one user who had dissociated accounts at the time of association. So it'll first check Stack Overflow, and associate with that if you have one. It'll then grab the accounts associated with that. I would expect that if it didn't find a Stack Overflow account, it'll then advance to Server Fault, then Super User, then Meta, then Area 51.