If the site has previously indicated to Safari that it wishes to always be accessed over HTTPS through HSTS ([HTTP Strict Transport Security](https://en.wikipedia.org/wiki/HTTP_Strict_Transport_Security)), then Safari will always try to redirect to HTTPS.

You can clear the HSTS cache by deleting `~/Library/Cookies/HSTS.plist`.

Note that Safari does also cache 301 redirects for a while and thus clearing the normal Safari cache may also be necessary: from the Develop menu (enable in Preferences → Advanced), choose Empty Caches.