Cause unknown, as some posters suspected. It would be interesting to know how frequently aircraft use full afterburner at low-level in the training area, and whether there is an unusually high incidence of it being employed through Star Wars Canyon.
Incidentally I was in the US when this happened and drove past the Father Crowley Overlook just as the crash investigators were finishing up around a week later. Flying activity was apparently diminished but still taking place.