When I had a look in the Top 10 Referral Sources reporting in the Website Activity Dashboard, I saw over 82% of the referral sources came from campaign. For this reason I contacted Optimizely Support team within ticket #1499366. They saw that every campaign event from campaign is getting in this referral analysis and that is the reason, why 82% of the referrals come from campaign. Especially the campaign "sent" event is tracked here. This makes no sense here. Only the "click" event should be tracked here, because otherwise it is not a referral event.