Complex systems tend to have complex control systems and feedback loops that compensate for perturbations and try and maintain homeostasis.
But when these feedback loops break, for whatever reason-- reaching the limits of the elasticity and controls, for instance-- they tend to abruptly de-compensate and fall apart.
This looks sudden: more and more slack is being taken up, until the actual compensatory mechanism is exhausted, and then smack, it's all over.
It's like when you get hypothermia, and then vasoconstriction to maintain core temperature stops... and you start shedding lots of heat through your skin, and shivering ends... then you feel hot and want to take off your clothes. Your body is doing many different kinds of things to try and fight getting cold, and once one falls apart "because it's too cold" -- the rest fall like dominoes.
That said, this doesn't stop someone from designing a complex system which cannot tolerate failure: it's just it'll tend to fall apart as soon as they start to put it together (and they'll either learn quickly how to make it tolerate failure or it'll never get off the ground), it won't generally run fine for a long time and then implode suddenly.