I'm not saying you're one of these people. But on the other hand I do get frustrated often by bikers. Maybe you did something you didn't notice or something. Still, they don't need to do stuff like that.
Like recently I saw a biker going fast around a roundabout, I have right of way there, but he didn't stop. So I sounded my horn and he just flicked me the finger.. If I had hit him he'd probably never lift his finger again.