Well here is one explanation, for what it is worth:
Precautions
Unripe mangoes should not be eaten in excess. Their excessive intake may cause throat irritation, indigestion, dysentery and abdominal colic. One should, therefore, not consume more than one or two green mangoes daily. Avoid drinking water immediately after eating the green mango because it coagulates the sap and makes it more irritant. Sap or milky juice which comes out on breaking the stalk of the green mango is irritant and astringent. Eating green mangoes without draining the sap may cause mouth , throat and gastro intestinal irritations.
