I've now had a quick check of links I might have clicked from this thread (or onwards), and none have generated a Breitbart cookie.
So I would suspect advertising. But I have an adblocker, and certainly haven't noticed anything looking like Breitbart coming up.
Interesting. I wonder where they're coming from?
BTW, if people felt able to state where their t.co and other shortened links went, I know I'd find that helpful. At least one link (added for good reason) went to Sputnik, so I immediately had to go off and clear its cookies. Not a dreadfully onerous task, of course, but sometimes I'd prefer not to stop and do so.