So, looking over it briefly, I'd suggest taking that page with a grain of salt as there appears to be some kind of issue with the data filtering. I'll try to look in to it a bit more and I can't go in to all the details, but mods have a bit more depth to our view of the analytics and our view and the 25k+ view don't match up... like at all...
Update: Ok, so I was able to reproduce the data that's visible there. The 25k+ screen is highly misleading as the "history" data lets you filter, but the referrers section is a fixed 1 month window that covers the last full month (ie, it currently shows the month of May.)
Now that I know that, I can tell you that the top referrer sites change pretty regularly. So far for June, workplace has a commanding lead with over double the referrals of any other single source. Why exactly that is I'm not sure as I don't have any more information on what the exact links are coming from. But it's highly variable whatever it is.
My best guess would be that maybe it has to do with hot network questions and which particular target audiences it strikes the fancy of, but it's really hard to say for certain as anything would just be a guess.