It is likely an anxiety response. She may be picking up on peoples stress and anticipation, and misinterpreting it as a reason to worry about loading. I would go back and make sure that she has positive associations with the trailer. You can use positive reinforcement to do this. You may also want to try loading her with things more similar to how they may be on a show day, ie. loading boots, sheet, other horses in the trailer, etc.
Try to get her really comfortable with loading through making it a normal or semi-normal thing. If you can, maybe try walking her on and off the trailer a couple of times a week during your normal routine, just so it becomes a more normal activity for her. It is a little tedious, but hopefully it pays off!