After years, months, or even weeks of heavy drinking, your body has become accustomed to this lifestyle and withdrawl symptoms when you either quit cold turkey or drastically cut back. Alcohol has a depressive effect on your system meaning it slows down brain function and changes the way your body processes the messages your nerves send throughout your body.
After a long period of time of heavy drinking, your body adjusts to constantly having alcohol in its system. The way your body adapts is by putting your brain in a more awake state to keep your nerves alert and talking to each other. When you cut back on the alcohol and the levels drop, your brain stays in this awake state which causes the withdrawl.