Why does an imploding tank car implode so quickly? Mythbuster Video for reference
In the imploding tank car story, hot steam gas at atmospheric pressure was put inside a steel tank car, and the valve closed overnight. Gradually the steam condensed and cooled down, severely lowering the pressure inside the tank, and then atmospheric pressure squeezed the tank car. 
But my question is, why does the implosion take place so suddenly and rapidly? Why doesn't the tank car implode slowly and gradually as the pressure continuously drops inside (ie. the dent slowly grows)?
Compare this with, say, an empty water bottle. When you suck the air out, the bottle continuously deforms and flattens out, it doesn't retain its shape and then suddenly collapse. How are the two containers designed differently?
 A: Tanker cars that carry fluids at normal temperatures and pressures are generally designed to withstand the outward pressure that the fluid exerts on the walls. The construction is usually cylindrical or ellipsoidal in shape. These shape help to distribute the hydrostatic forces evenly and eliminating sharp corners reduces the likelihood that pressure stresses will concentrate over small areas.
But if you empty a tank of the same construction, seal it and provide any means to evacuate or reduce the pressure within, there are two factors that can lead to sudden collapse. (1) the vacuum pressure force is in the opposite direction that the normal hydrostatic forces impose on the tank and (2) the vacuum forces could conceivably reach greater pressure than what the hydrostatic forces reach (up to 14.7 psig). If the tank uses external gussets that reinforce the strength of the tank, the gussets no longer assist in supporting the tank wall under vacuum since they act to support in the opposite direction.
But to answer your primary question, all it takes is one minor deflection, fold in the tank wall, the even distribution of pressure is lost and all of the sudden stress is concentrated at that one point. This leads to rapid escalation of the collapse.
