Why unexpected attached files are loaded in all mailings by default?
I think I've found the cause. Check out bug CRM-16544, it matches your problem exactly (except theirs was Grant and yours is Event). I think we can fix this by adding civicrm_event to the array on line 210 of Container.php, but I'm not familiar enough with the Symfony stuff to be sure. I think your best bet is to file a bug.