I do understand what is happening here.
Some websites do provide previous /next buttons for page navigation and some don't.
In absence of Previous/Next buttons, a user tries to use the back button.
Upon clicking the back button, aforementioned dialog drops down, creating a loop.
This is to prevent submitting the form multiple times.
If this happens on a site where payment is involved, it may produce a duplicate charge if a user submits the form again..
This issue has been around for sometime.
Apple hasn't published a knowledge base article about this.
Until then, navigate away from the site by visiting another site.