You could be right about the reed, though of course I didn't see how the circumstances were. If there was wind and we can see from the photo that the horses were in full throttle, there could be a momentary dry reed stuck on the front element only to fall away at a later photo.
I know something of the like can happen and it doesn't even have to be a bulky piece of reed. I have long hair which can get in front of the lens in strong or mild winds and even one single tress can have this effect. I have several good photos that have these diagonal bands at different angles, all running evenly across.
All that said, I hope no other technical problem is there for your camera.
ps. I can show some examples from my archives if you feel you need some reassurance.