Hey, Runatic!
This unwelcome plot twist rarely has a single culprit.
A scientific review describes a combination of mechanical jostling, changes in blood flow to the gut, and diet. There isn't enough research to prescribe the same list of forbidden foods to every runner. Review of runner’s diarrhea
The authors specifically note insufficient evidence to recommend a gluten-free diet to prevent the problem in runners without celiac disease.
One practical detail: test your race breakfast in training. Someone else's tried-and-tested oatmeal hasn't been tested by your stomach.
A marathon has enough surprises already. Breakfast can qualify in advance.