Magento 1.9 Shopping Cart – Why Does Shopping Cart Get Empty After Sometime?

Use an event observer to empty cart on customer login.

config.xml

<customer_login>
  <observers>
      <foo_bar> <!-- foo_bar should be unique tag for within all customer_login observers-->
          <class>foo_bar/observer</class>
          <method>customerLogin</method>
          <type>singleton</type>
      </foo_bar>
  </observers>
</customer_login>

Observer

class Foo_Bar_Model_Observer {
    public function customerLogin(Varien_Event_Observer $observer) {
        foreach( Mage::getSingleton('checkout/session')->getQuote()->getItemsCollection() as $item ) {
            Mage::getSingleton('checkout/cart')->removeItem( $item->getId() )->save();
        }
    }
}

Yes this is the default feature of the Magento. whenever you place on the Place order button on website it will create order and your cart becomes clear and redirect to the payment method page and their comes two scenario basically.

1. Your payment success. once your payments gets success payment method will return success message params to the checkout_onepage_controller_success_action and updates order status as success.
2. Your payment failed or you canceled on the payment gateway page Once you either cancel payment or your payment gets failed then your payment gateway returns to the checkout_onepage_controller_failure_action with response and updates order status.

So in both cases order gets created and your cart gets empty.

So if you still want to keep the products in the cart if your payment gets failed then you can customize the code with following steps

Write the observer checkout_onepage_controller_failure_action event and find the order id and get the product ids from the order and add this products programmatically to cart then its done