There has to be some sort of explanation as to why it happened. I too, was in complete awe. My analytical mind did something similar to some of these responses the first time I observed it.
Reality is that it can happen, until exposed to these situations I would have not believed it either.
My speculation would be that it would be a high heat situation in the catalyst followed by a misfire. Uneven pressure pulses (possibly vacuum) from the lack of combustion combined with a hot and weakened Cat material would allow the material to travel backwards for a short period of time. BUT, I'm just a mechanic.
@matreyia are you getting or did you get new turbos and oiling system components when they replaced the engine?
The whole shabang was replaced...turbos and all. The turbos were shredded.