The coupon code doesn't get reset from clearing the cart. If you want to clear the session variable when the clear cart button is pressed then you can do that by changing line 19 on your cart page from:
if((((isset($_POST["txtPromoCode"]))?$_POST["txtPromoCode"]:"") != "")) {
to:
if((((isset($_POST["txtPromoCode"]))?$_POST["txtPromoCode"]:"") != "") || isset($_POST['boh_Clear_100'])) {