New members are somehow automatically added to a different subsite

I am using Buddypress registrations and the plugin Multisite User Management. Sometimes (not all the time) a new user gets their own blog as an administrator, but also is added to someone else's blog as a subscriber. This problem causes their blog link to be listed to the wrong blog. Any ideas on how members can register and be forced to be a member of their own blog only?

Exception: all members become subscribers on the main site---this is fine.