A few months ago, many publishers and webmasters began noticing a huge spike in referrers from hsrd.yahoo.com.
Yahoo told me that this switch began to happen when Yahoo went SSL across their properties.
The hsrd.yahoo.com referrer is similar to r.search.yahoo.com, which publishers will also see. It processes click actions for the home page.
As part of our move to HTTPS secure search, referrer logs are now showing hsrd.yahoo.com when the source site was HTTPS-based.
So if you are seeing a huge change in referrer data from Yahoo, that is why.
Hat tip to Ben Lundin for spotting this and sending it my way.