I can see why you might've fallen over...
You should've put your non-dominant leg back and rested your weight backwards. This allows you to get a faster start, because of all the momentum created when you move at the buzzer: First, you rock your weight forward, creating a bit of inertia forward, as well as into the ground. Next, you push with your dominant leg, propelling you upwards, to counteract the downwards motion, as well as forward, adding to your velocity.
Also, it's harder to fall over this way, because you'll immediately catch yourself with your other leg... Well... usually.
The way you were standing, it looks like someone could've poked you in the rear and you would've tipped over forwards.
