Gastric dumping syndrome

A 35-year-old man comes to the emergency department because of fatigue, dizziness, watery diarrhea, and abdominal cramps. He has had episodes of these symptoms for the past two days, which have occurred within an hour after eating large meals. He says that he underwent a gastric bypass surgery three days ago with no complications except these odd episodes. Pulse is 105/min, respirations are 18/min, and  blood pressure is 105/75 mm Hg. Which of the following is the most likely mechanism for his symptoms?