WTF: blue Ford Explorer sitting on his horn

There's a guy who's been driving a blue Ford Explorer (with tan trim, roof rack, PA plates - I can give the # if anyone's interested) slowly up and down St. John's for the last half hour, more or less sitting on his horn. Any idea what's going on, or what's his damage? It's not even 9AM, on a Saturday. WTF?

Wow, I live at St Johns and Underhill too, but I didn't hear a thing. My apartment is in the back of the building though. And all my windows are closed (for the AC).