Set Default Category to Username

The default category is stored in the options table under option named default_category , so you can do something like this:

add_action('admin_init','set_user_default_cat_wpa89057');
function set_user_default_cat_wpa89057(){
    global $current_user; 
    if ( isset($current_user) ) {
        update_option( 'default_category',  $current_user->user_login);
    }
}