MyBoyPuck -- look carefully at the interior of this trailer. What colour is it? How many doors can you open during loading? What is on the floor?
If it is only this trailer, then there is either something different about it, or something happened the time before that has made him nervous. Could be that it makes a different noise when he loads? Something jingling that's new?
My mare also will load fine, then back up, load again, back up... she would do it all day if I would let her. Last time, every time she backed up I worked her then loaded her again and brushed her. But, frankly I don't have the patience for this nonsense. Next time we go somewhere, I'm just going to back her into the trailer. I know that her issue is that she doesn't like her head in the confined front of the trailer.