the add to cart button creates a session named WAEC_ContinueRedirect that is used by the continue shopping button.
you could add the following code to line 1 of the cart display page to set that variable if it has not been set yet:
<?php @session_start(); if(!isset($_SESSION['WAEC_ContinueRedirect'])) $_SESSION['WAEC_ContinueRedirect'] = "/index.php"; ?>