Well, let me tell you a little story....
Standardbred barn. Filly was there for starting and was supposed to go back home. The grooms tried 3! Weekends to load her....with halter, in driving tack, drugged up to her ears.....nothing. Not one step forward in the trailer. After watching the drama I went to ask if I could help(letting the men try first, of course, what does a woman know after all
The trailer had no rubbermats, just bare aluminum floor. So I suggested to put some straw down. And on she went.
So, could be that your horse slipped. Not saying that you have no mats, lol, but a little straw or shavings might help. And an opening on the other end. And lights.