It's not about whether it was silly string or a rock, it's about a little something called the Startle response. It's a defensive reflex that responds to sudden and unexpected stimuli, whether that's soft caress or a punch. To protect the back and neck it creates a whole body startle (a jump or tensing of the muscles) and to the protect the eyes it causes you either blink once or close your eyes entirely.

So imagine you're riding 40km down a steep hill and suddenly something hits your face, without even thinking about it you jump a little out of your seat and shut your eyes briefly. because you were startled you fail to notice a pothole directly in front of you and go flying over the handlebars.