Funny, it is for me: any wikipedia page redirects to its https counterpart.

E.g. http://en.wikipedia.org/wiki/Hacker_News redirects to https://en.wikipedia.org/wiki/Hacker_News .

Do you have a user account at Wikipedia by any chance? They've been redirecting logged-in users to the SSL version for a while, and under some circumstances it also seems to redirect users who have been logged in but aren't right now.

I do indeed have a WP account. It redirects as well when I'm logged out.

Are you using the HTTPS Everywhere extension, perhaps?

No I'm not. I'd love to, but there is none for Safari as far as I'm aware of.