loadUserByUsername has some issues that I can see.

GithubAuthenticationToken authToken = (GithubAuthenticationToken) SecurityContextHolder.getContext().getAuthentication(); failes with class cast exception when an anonymous user is accessing. It should throw a UserNotFoundException for anonymous users I believe.

Also, possibly when the github token has timed out and is invalid, the plugin does not redo the login. I'm pretty sure its failing in loadUserByUsername but I'm not totally sure. Will update if I find out.