Fat in the abdominal region (beer belly) is an indication of "insulin resistance". Insulin resistance is metabolic syndrome that is a precursor to diabetes (which we know is a dis-regulation of blood sugar). I wrote about insulin resistance here...
Insulin resistance is caused by the over consumption of sugar and refined carbohydrates. This floods the bloodstream with glucose. And the body has to respond by flooding the bloodstream with insulin to shunt the glucose into the cells. The problem is that when there is too much, the cells resists and no longer listens to insulin. Hence insulin resistance.
Because high glucose is toxic, the body needs to get it out of the bloodstream. It can no longer get it into the cells. So it converts the glucose into fat which is stored in the abdominal region.
Also see "Abdominal obesity" in Wikipedia.
Why some people get fat and other do not depends on individual genetics and amount of sugar and carbohydrate consumed. It also depends on age. Older means more susceptible to insulin resistance. And many other factors including exercise, etc.
So beer belly is not cause entirely by beer. As there are people with beer belly that do not drink. Other call it "wheat belly" because gluten sensitivity may also cause it. Gluten is a protein found in wheat and anything made from wheat such as bread, pasta, processed foods, etc. Look up the book "Wheat Belly"
So to avoid beer belly, you should avoid sugar, processed carbohydrates, gluten, wheat, bread, pasta, pastries, and processed foods.