It has to do with your liver. Normally, when your blood glucose level starts to drop, your liver steps in. It goes to work changing stored carbohydrate into glucose. Then it sends the glucose out into the blood, which helps you avoid or slow down a low blood glucose reaction. However, when alcohol enters your system, this changes.
Alcohol is a toxin.
Your body reacts to alcohol like a poison. The liver wants to clear it from the blood quickly. In fact, the liver won't put out glucose again until it has taken care of the alcohol. If your blood glucose level is falling, you can quickly wind up with very low blood glucose. When you mix alcohol and exercise, you increase the risk of going low.