Not a clue how this happened. I just opened the trailer for the first time this season after being closed all winter. It's sitting alongside my garage......outside all winter, uncovered. With all the snow around it all winter, I haven't checked it at all except what I could see from the road. We have had some storms blow through recently.......something might have hit it and then blew away. No evidence of anything inside the trailer.
I'm going to try to take out the frame of just the broken section and bring it to a glazier I found for the replacement.( that is if it ever stops raining long enough here in west Michigan!)