Wouldn’t have necessarily broken at the same time, despite it seeming to do so.
You could imagine a scenario where one chainstay has failed, but failure is not known to the rider.
The rider then rides bike with failed chainstay up a 10% grade, stands up and kicks. Second chainstay fails. Bike is now supported by seatstays only.
At this point with zero chainstays in play, both the seatstays snap off like twigs.
To a rider this would seem like it all failed at the same time, because the process of the chainstay snapping and then seatstay snapping might happen in under a second.
You can imagine variations on that scenario but it would seem likely that one chainstay was compromised or failed, which led to the chain reaction of events that made it seem like the back of the bike fell off in one fell swoop