What is rewriting my user related URLs?
The module that creates a URL like localhost:8888/mysite/?q=user/bwadmin (or localhost:8888/mysite/user/bwadmin, with clean URLs enabled) is the Pathauto module, which creates path alias for user, node, and taxonomy term URLs.

Check the module settings; there is a set of options that allows replacing some specific characters that would cause some problems; one of those characters is the underscore. In your case, the module is set to strip off underscores from the path alias it generates.