It will make the community activity sub tab as default activity tab on profile

Community activity as Default

//fix the nav default
function bpcom_fix_nav(){
 global $bp;
 if(!bp_is_my_profile())
     return;
 if ( $bp->current_component == BP_ACTIVITY_SLUG && $bp->current_action=="just-me" ) {
     
     $action=bpcom_get_current_actual_action();
     
     if($action==BP_ACTIVITY_SLUG||$action==  bp_get_loggedin_user_username()){
         
         //unset the default settings
	remove_action( 'wp', 'bp_activity_screen_my_activity', 3 );
	add_action( 'wp', 'bp_community_activity_screen', 3 );
	$bp->current_action = BPCOM_ACTIVITY_SLUG;
         
         
     }
	}
        


}
add_action("init","bpcom_fix_nav");

function bpcom_get_current_actual_action(){
    $request_uri=$_SERVER['REQUEST_URI'];
$uri_components=  explode("/", $request_uri);
if(empty($uri_components))
    return false;//just to prevent infinite loop

while(!($current_action=  array_pop($uri_components)))
    ;
return $current_action;//first non empty string

}