If you weren't intending to visit this site (as you said):
Start by clearing your browser cache and cookies, including 3rd party cookies and restart your browser.
If that resolves it you should be good to go.
If it doesn't try running your browser with add-ons disabled.
If that resolves it, have you added or updated any add-ons ?
If so try disabling that add-on - and restart and try again.
EDIT: I have only noticed that you are posting in the Avast Mac Security sub-forum and with Safari. Whilst I don't use either of these products, you should still be able to do this in Safari.
