I *think* it might be because you have a javascript in place on your index page that redirects to the shop directory
Code:
<script language="Javascript">
<!--
var URL = "/shop"
var speed =00000
function reload() {
location = URL
}
setTimeout("reload()", speed);
//-->
</script>
I am by far no expert LOL but I am thinking given that any visitor is redirected to the shop directory, whenever they click the back button it would just take them 'back' to the main index page which then again redirects to your shop directory.....if that makes sense LOL
There are other methods of redirecting, including meta refresh and .htaccess plus others :)
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