I've been having this problem for a few months now.
I run a blog where I have 17 writers subscribed and constantly posting. When I go in to edit one of their posts for whatever reason, it automatically makes me the author of the post and doesn't give me an option to change it at all.
I'm going to go ahead and say that it's the plugin 'Members' that I have installed on it and if it is, I would like to MAYBE find a way to keep the plugin and fix this issue.
Of course, uninstalling isn't critical, but I really like the options it has and how it works.
Am I just not realizing something extremely easy that's right under my nose?