It's not the case that the crew members of the Columbia died from any one specific event, nor could we know that, anyway. As NASA's official 400-page report said, they died from a confluence of "lethal events" as their shuttle fell to Earth. The first event was "depressurization of the crew module," meaning that the shuttle ripped open and lost air. This would have made the crew members lose consciousness on the spot. Then, the module started spinning — the second event — which might have been alright except that the shuttle didn't come equipped with head restraints. In other words, unlike a roller coaster there was nothing in place to protect their necks.
At that point, any crew members remaining alive would have been subjected to the third lethal event: "separation from the crew module and the seats with associated forces, material interactions, and thermal consequences." That is, the crew flew out of their seats, or the seats broke off from the module while they were still seated. NASA described this event as the "least understood," and recommended future shuttles be designed to allow for the least possible catastrophic disintegration if some similar disaster happened.
With all air gone and life support lost, the crew would have asphyxiated and been subjected to the kind of atmospheric temperatures and conditions that a human body was never meant to endure. Finally, no one could have survived the impact with the ground. That was the fifth and final event.
