Or, could be something entirely different

Or, could be something entirely different, such as the Grapefruit-Seed-Extract killed some kind of parasite or bacteria (or candida perhaps) quickly, and the body responded by eliminating all the dead matter quickly.

Someone on another forum made and used liposomal vitamin C, which does not have any of the preservative you mentioned that is sometimes in GSE. The person who tried a large dose of liposomal vitamin C also got diarrhea within an hour. Perhaps the liposomal delivery system brings the vitamin C or Grapefruit-Seed-Extract to areas of the digestive system (liver, etc) that would not be reached if ordinary vitamin C (or ordinary GSE) are taken, thereby killing off some kind of undesirable bug there. Thus causing diarrhea.

Just another theory.