Some people can't work out the dynamics in their brain when reversing the car. Rational thought process goes out of the window and they seem to freeze.
They know deep down that they can stop at any time and quickly check mirrors, look behind to judge distance etc., but the act of actually moving backwards seems to override everything and they seem genuinely frightened to move more than about a foot.
There needs to be a reversing part in the test which covers this type of thing, as I've seen it where the car actually hit another car, and nearly ran down a passing pedestrian.
Another occasion, a lady couldn't work out the reverse turn dynamics of the steering wheel when going backwards, and kept angling more and more towards a massive shop window. Yes, she had manouvred herself up onto the pavement by then.
She tried another time and ended up completely nose first towards the shop, precisely 90 degrees to the window and the opposite of what she intended.
She burst out crying and an off duty policeman ended up helping her out of the car and parking it correctly (parallel to the kerb) for her within three or four shunts, taking about twenty seconds.
It would be interesting to wire the brain of one of these drivers and see what is going on. Or not, as the case may be.