I was curious about this myself and started by looking in Activity Monitor, found that "launchd" was the service that kicked off the Yahoo Contacts Sync. After some googl'ing I found out that custom scripts for launchd get executed from the ~(user)/Library/LaunchAgents folder. In there, you'll find the culprit -- at least, I did. Now I dunno if I should simply delete them (since I don't care about syncing contacts, I don't use the Mail.app) or if I can just disable it by tweaking the plist file. Hope that helps!
EDIT: I discovered that going into the AddressBook.app preferences and unchecking the accounts listed caused the plist files to be removed. If you check them again, they reappear. Works for me! :)