Wassat, just wiggle the bars when approaching a turning cage?
The detailed report below goes over it in detail but basically if you look at a car and the background isn't moving behind them then you are also not moving in relation to your background. Our brains are tuned to take notice of things that move against the background and we may overlook something that isn't moving across it. This is especially true in a situation where we are taking a quick look. A little bit of sideways motion helps us break out against the background and be more likely to be perceived
. (The eyes might see something but if the brain does not perceive it as a threat it might ignore it altogether and not let the driver know something is approaching.)