I wonder if it has to do with how you fuel during the race. For my trail marathon I was able to snack on solid food at all the aid stations which were between 4 and 6 miles apart most of the way. In between that I used my homemade iskiate which has never been an issue for me. I was good for that race. But, my recent 50k that I completed was a little different. I was "running loose" (luckily not explosively loose, hahaha!) the whole way. The only thing I can think of that was different (because I had almost the same exact dinner the night before as my marathon) was that I wasn't able to snack on solid food during the run. There was only one aid station in the middle of an out and back course. I think having the gel with nothing solid had my body all messed up. I don't know. Just a thought.