I doubt its the internal pressure, if it was, the engines wouldnt last very long.
The engine is the tallest part on 1/8th scale cars. With the wedge body, when one car hits another, the car that was hit probably hits the engines head, and thats when the crankcase breaks IMO.

You also have to take into consideration the inertia forces invloved, the car that is coming down the straight stops pretty quick when it hits the car other car.

It would be neat if some RC car company would be willing to donate a car to see how hard a car traveling at those speeds hits. I've heard of people breacking legs when they get hit.