By the look of things, whatever happened, prompted the crew to leave cruise alt immediately, but left the crew incapacitated in seconds, as they couldn’t execute any logical subsequent actions, to save their pax (and their own) lives.
Rapid decompression due to structural failure (or even a collision with some FOD dropped from another passing plane) could explain this scenario.
One of the possible keys to unlock this mystery is the reaction of the pax and cabin crew; in 8 long minutes, were there any attempts made to call or text somebody? Or the event that rendered the crew incapacitated also affected the pax and cabin crew at the same time?