It was a crude but brilliant device, designed for mass murder. Or the carrier may have been an innocent - carrying a present. Alternatively, it may have been an unaccompanied device. It may somehow have been smuggled airside and slipped into the container of New York luggage. Either way, once the plane took off for London, things started to happen.
Technology makes tragedy A simple plan with deadly results Modern airliners are pressurised.
The outside air is much too thin to breathe, so the air-pressure inside the fuselage is higher than outside. But it isn't sea-level pressure. The pressure is less than it would be at sea-level, but more than it would be outside. The moment the pressure dropped to 8,000 feet, the barometric sensor completed a circuit which started the timer. We don't know for sure how long a delay had been set, but it was probably about 4 hours.