When you try to maximize your stopping by using front and rear both (or if you use just front) the weight shifts toward the front, which is what makes the nose go down.
At that point it's real easy to skid even a drum rear because there's less weight on it than usual.
The extra weight on the front makes the front harder to skid, which is why they put a strong brake there. If you do skid the rear and it's an agressive stop, you're in danger of having a high side crash. At that point either of two things can cause the crash; either the rear slides out to the side more than the front wheel will turn to keep the bike going straight and the bike flips, or the second thing, if you make the mistake of getting off the rear brake to stop the skidding out to the side, the rear straightens up so quickly when it starts turning again that it bucks you off.
The rear can go out to the side even when you were braking in a straight line because the rear of the bike wants to go faster than the front, since the weight transfer is giving traction to the front, and the rear is skidding giving it comparatively little traction. The way to straighten it up is to get off the front brake for an instant, which makes the front of the bike want to go faster than the rear, dragging the skidding rear back into line. Some people even recommend pressing harder on the locked rear at that time to make sure it doesn't start turning when releasing the front causes weight and traction to shift back toward the rear for an instant.