bp_load_theme_functions()

function bp_load_theme_functions() {
      global $pagenow;

      // Do not include on BuddyPress deactivation
      if ( bp_is_deactivation() )
          return;

      // Only include if not installing or if activating via wp-activate.php
      if ( ! defined( 'WP_INSTALLING' ) || 'wp-activate.php' === $pagenow ) {
          bp_locate_template( 'buddypress-functions.php', true );
      }
}

If you need to stop BuddyPress from loading the buddypress-functions.php file use the remove action below. This is useful if you are supplying functions with your custom BuddyPress theme that may interfere with the core functions.

remove_action( 'bp_after_setup_theme', 'bp_load_theme_functions', 1 );

Source File

bp_load_theme_functions() is located in bp-core/bp-core-template-loader.php