Since there is really nothing in nuts that causes diarrhea, if you have this reaction after eating pecans it is likely that you have experienced an allergic reaction. If you find that you get diarrhea after eating pecans, it will be important to avoid them altogether.
Although most food allergic are usually not life threatening, sometimes they can be in certain rare cases. At the very least you will definitely not enjoy the allergic reaction that comes along with eating certain foods that the body is allergic to. Nut allergies are extremely common so it will be important to pay attention to any of the warning signs so you will be able to steer clear of these nuts or any other foods that you may be allergic to.