This code is probably not the problem. The form action for the checkout page is the confirm page. It is the only page that would send you to the checkout_failure.php page, so the fact that you are getting to that page means that the problem is on the confirm.php page.
You should be looking at that page and the code that processes the authorize.net payment. Specifically the trigger in the UI or the IF statement at the very top of the autorize.net code.