I think why some people dump & some people don't is one of those million dollar questions! It may have something to do with what you have eaten throughout the day already. If you have eaten low-carb, low-sugar, low-glycaemic foods, your blood glucose levels would not rise so high and would then subsequently be returned to normal levels more quickly after eating (typically around 2 hours). If you eat a high-carb, high-sugar or high-glycaemic meal, it can take the body much longer to reduce & stabilise blood glucose levels, so if someone eats a small bowl of ice-cream an hour or two after eating, it could just tip the blood glucose levels over the edge...and bingo you dump (and I'm not suggesting anyone is eating badly here...it's just that we all know foods can vary greatly in ingredients).
Also, we are all very different and we will each metabolise foods/drinks at very different rates. Other factors may include any medication we may be on, whether we have any diseases (rate of metabolism in diabetics would be very different from non-diabetics), how much we exercise etc.