Obviously something was thrown by the blades hard enough to crack the oil pan.

I had an old Ford redbelly or two and the oil pan WAS the tractor. You could remove the engine and push them around with only the oil pane holding the front end to the transmission.

Those things were indestructable but unfortunately obselete. I replaced mine with a Kubota that I treat very tenderly because they are far from indestructable.

Bush hogging is a dangerous job. Something can crack the back of your head just like it did the oil pan. It have heard all kinds of storys like the guy that got a piece of barbed wire through his chest.