What is blocking referer headers?

I posted something to Show HN earlier today and my analytics dashboard (Simple Analytics) tells me that ⅔ of the visits are direct / no referrer.

At first I though they must be running in Incognito/Private mode, but I tested Chrome/Firefox/Safari and they all still send the referer in the request header even in Incognito/Private mode, which seems odd to me as that would be an easy thing to hide.

So what would cause this? Ad blockers? Corporate firewalls and proxies? Or are ⅔ of you copying and pasting the links on HN instead of just clicking?