Judging by this photo of the wreckage (chord wise crushing of the wing etc.) it looks like the plane went in almost vertically. Low altitude maneuvering (aerobatics?) can be rather unforgiving of mistakes. Stall ..... ? On the other hand, the pilot was a crop duster and was probably a past master at low altitude abrupt maneuvering.
Whatever the cause, it's a shame; however, it's fortunate that on one else was injured.