In an ideal loading environment, at or above dock level, the load back rest supports the pallet load when entering and exiting the trailer; conversely, loading down into a trailer takes away this support when entering and exiting.
Trucks coming in with low floor heights often have the additional disadvantage of low top clearance. Not only does this decrease the available top clearance for equipment entering the trailer it also creates a gap between the top of the truck and dock seal header which allows rain, dust, & outside temperatures to affect the loading process.
The risks are not always readily apparent, but rain and oil combined with wood floors make for a slick operating environment. In these conditions operators need sufficeint speed to climb out of the truck, risking dumping a pallet or worse trapping your equipment in the trailer.