Imagine you are always driving your car with the parking brake on. The parking brake depresses (lowers) your speed. You adapt by pressing harder on the gas. One day you take the parking brake off, and because you're still pressing hard on the gas you lose control and speed into a brick wall.

Similarly, the brain adapts to alcohol (the parking brake) by removing some portions of the overall braking system (inhibitory GABAa receptors) to compensate. When the alcohol is suddenly removed, the usual GABAa brakes are so sparse that the brain speeds out of control into a seizure.