The tailgating vehicle is a hazard to you because braking suddenly may

cause your brakes to fail

result in the tailgater hitting you from the rear

distract the driver ahead of you

result in the tailgater getting a ticket

Based on the given scenario above, when you break suddenly, when the tailgating vehicle is a hazard to you, it could cause the tailgater hitting you from the rear because tailgating is where the vehicle is driving too close to you and by that, when you hit the break, the car behind you may likely be surprised and hit you within instant without having any warning.