Eu não vi o filme, mas de acordo com Wikipedia
Flight 7500 suffered a catastrophic decompression, communication had been lost, and a fighter jet has confirmed the death of everyone on board.
De acordo com a Moviepooper:
A bout of turbulence shown earlier in the film, caused the breakage of a critical seal. This damage caused an immediate decompression of the entire plane. The oxygen masks malfunctioned which lead the immediate deaths of the flight crew and all passengers abroad the plane.
Um piloto licenciado contou ABC News que quando a cabine de um avião despressuriza:
"What happens first is your brain is not as quick. You’d get very
confused and stop thinking clearly,” he explained. “Then you’d fall
asleep and eventually die due to lack of oxygen."
Basicamente, a pressão fora do avião é muito menor, de modo que todo o ar sai da cabine (para alcançar o equilíbrio com o ar externo).