I found something.

There is a stainless steel roller where the film goes in. It is supposed to keep the film from unspooling. The top edge of the mysterious exposure matches with this piece. I had to load the film under shade but nevertheless outside. I wonder if the film got unspooled and buckled just so that this exposure took place when I loaded the film, and just happened to land where it did??

I just rolled the film back into the frame again and when the arrow is matched, frame 1 is just 4 layers behind the backing paper. With some light piping happening before frame 1, that may be the cause...