bbPress will use that forum.php file as a wrapper template for all of bbPress pages and that is most likely what the issue is.
I’d suggest if you need a wrapper template to work with your theme you copy the contents of your themes page.php to a file named bbpress.php.
Then copy any bbPress templates you want to customize to your themes folder.
See these for more info on copying bbPress templates and child themes:
http://codex.bbpress.org/functions-files-and-child-themes-explained/