Exiting the spacecraft involved releasing the pressure then pulling the hatch inward. But during a “plugs-out” launch dress rehearsal (where the spacecraft was unplugged from support systems and fully powered up to test everything except the actual liftoff), a fire broke out. An oxygen-rich environment, plus combustible nylon and Velcro (used to hold equipment in place), caused the flames to spread rapidly. The heat from the fire increased the air pressure in the cabin to the point where it was impossible to open the hatch.
