Could be wrong but Im sure tests shown that horses prefered to travel facing the back, hence the craze for rear facing trailers etc. I have read that if loose some horses turn round in a trailer to face the back.
I heard it was easier for them to balance backwards and also if you brake hard, they prefer to lean on their butt than their chest. Ive also heard its better for their feet, but not sure about that one - something about their pedal bone rocking the wrong way.... I cant remember so will stop right there!!