When working with Mr. Covey, how does Fredrick almost get killed?

Frederick had an accident with an oxen cart.

I had never driven oxen before, and of course I was very awkward. I, however, succeeded in getting to the edge of the woods with little difficulty; but I had got a very few rods into the woods, when the oxen took fright, and started full tilt, carrying the cart against trees, and over stumps, in the most frightful manner. I expected every moment that my brains would be dashed out against the trees. After running thus for a considerable distance, they finally upset the cart, dashing it with great force against a tree, and threw themselves into a dense thicket. How I escaped death, I do not know.