Actually its because of the wiring. Your brake light bulbs double as turnsignal bulbs. Your turn signals are hooked up to the same circuit as your hazards. Since you dont have separate turn signals when you hit the brakes you complete a shorter circuit with less resistance because the lack of a flasher causing just your brakes to light up. Other trucks for instance the blazers have two sets of lights. Brakes, and then turnsignal, hazards are the second. Those will blink when you hit the brakes. Just the way its wired.