Sometimes your hair or hat can look like a tasty meal. Sometimes it's territorial, if you're getting too close to a nest or whatever. Usually it's one of those two.
A long time ago, when I had much longer hair and would go running just about every day, there was a block on my route where a bird would swoop down constantly and attack me. I think it thought my hair was a bunch of worms or something. It was a smaller bird, it couldn't really hurt me, but fuck that thing flying around my face was terrifying. I quickly changed routes once I realized it wasn't going to stop.