From the 'beach' marks on that fracture surface, that crack had been growing for some time and somebody was not looking
I think you'll see on closer inspection that the crack originates internally, and would not have been visible.
I guess from your picture the aircraft landed OK
It happened about 100 ft on final to a grass field. By the time it landed, the upper tang was bending upwards and the aircraft was jumping around uncontrollably. The only thing keeping the outer portion of the tang attached was the friction from the washer.