Website forces user to click back button quickly and repeatedly to go back

So I built a website that provides link to an external website. On the external website, when the user tries to go back, it won't work unless the user furiously presses the back button multiple times. It's not a browser problem so my guess is that it's probably on the external website's side. The URL will switch to 'http://website.com/index.htm', with '/index.htm' at the end, then revert back to what it originally was. Would anyone know what could be wrong?

Thanks