• When your site is on HTTPS and you are linking or redirecting to a HTTP site, the browser will not send a referrer.
• This means the target site will see your traffic as "direct traffic", i.e. they cannot distinguish such hits from a user who directly typed in the URL.

Reasons for this behavior

It's probably because of this RFC Show archive.org snapshot :

Clients SHOULD NOT include a Referer header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol.

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Read more Show archive.org snapshot

It's not clear why this SHOULD exists. One might think it's because of session IDs encoded in the URL (something that was in fashion in the 90s), but then again browsers do send referers when linking from HTTPS to HTTPS.

Fixes

• Tunnel links through a non-HTTPS page (but test the effect because HTTP referers survive a 301 redirect)
• There's a new <META> tag Show archive.org snapshot you can use in some modern browsers.