Best practice when doing user research with surveys?

Could you suggest best practice when doing user reasearch with surveys with open questions?

Context a web page has (unexpectedly) a lot of visitors we know nothing about. This page is not listed in the navigation and there were no SEO efforts to improve its ranking in search engines. The topic of the page is a common topic also for other pages.

We were wondering why does this page have so many visitors. One idea was to ask a single open question in a survey in order to find out what the visitors are looking for. The survey will be integrated on the page (as a widget) or will be shown as a popup. The exact implementation is yet to be decided. Is there a best practice you can suggest for this scenario?