At the time of the accident, there was a huge, fully laiden trailer attached to the back of the tractor. The photo you have was taken after they unhitched the trailer. There are more photos of this accident while the trailer was attached.
Story I heard was the guy was ridin with his young son in the cab. Driving at speed with a heavily loaded trailer behind. The son accidentally pushed the fel control forward causing the fel to dig in. Carnage ensued.
Ouch. No one was seriously hurt afaik.