Some time we face the 503 error in Magento website. Reason behind it is Magento Downloader. If you will check Magento Downloader you will get a option to unable or disable the maintenance mode. You can go to Magento Downloader directly by adding “downloader” after your domain name. E.g. http://www.yourdomain.com/downloader
Sometime it can be done because of downloader script timing out. It this case, we should go to downloader are run the script again or download the full package instead.