It's not obvious what caused the failure. As you can see she has a plow, so perhaps it was just too much loading.
Not sure what part failed first? Silent Chain? input/output shaft chain teeth? Ring and pinion drive? If I had to bet I'd say that one of the teeth on the input/output shafts came apart, then as it bounced around the tranny just started causing carnage. The teeth actually came off whole, like you could just hotmelt glue them back on
