She finds this causal explanation to be the key in repetition of the same accident over a period of time.
Similar story happened at NASA in the catastrophic failure of Challenger Shuttle. NASA was cognizant that the launch conditions were not right but assumed that as nothing has happened in the last 25 flights, nothing will happen. The crucial gut instincts of engineers were ignored by the decision makers, the engineers could not voice their gut instincts strongly due to lack of sufficient data in an organization whose ethos required substantial data to justify something.
The successive commissions fell into the same trap, reconstructing the events that led to the catastrophe in great detail. They gave too much emphasis on technicalities and highlighted the crucial decisions which went wrong and made individuals accountable and gave the system a quick fix. Vaughan took the different approach and rather than blaming the individuals primarily, blamed the organization.
To reach a conclusion she based her judgment on her already developed theory of normalization of deviance in organizational cultures. Keeping the model in front where she situates individual actions and tries to find their meaning in the culture of organization, she was able to ask the right questions and find those hidden clues which the previous commissions failed to observe. One reason why she was able to find something which remained hidden from the previous commissions is because she looked for them in the first place.
Vaughan argues that NASA managers did not break any rules unlike reported before rather abided by them a bit too strictly. She has pointed out that rather it were the rules that kept on getting more and more accommodative each time a mission was accomplished successfully. Thus rules expanded like a rubber band, enclosing more and more dangers which previously turned out to be false assumptions. It was NASA's culture where something which has worked once was assumed to work every time. O-ring seals were not a new problem but the previous successful launches prepared the managers' mind that it is an acceptable risk. She argues that rather than developing new paradigms for unexpected consequences, people change the paradigms to accommodate the consequence or discard them at all. According to her, the same happened in the case of O-ring erosion where the risk level was reduced as the risk did not turn out to be as grave as previously thought. However the reduction in gravity did not mean that the risk cannot become grave once coupled with some other risks. The O-ring erosion, sub-optimal temperatures & unexpected wind shears, all risks were expected to be less than grave, in the original paradigms but people failed to develop new paradigm for the combination. She has agreed that Challenger's Disaster was a normal accident (p. 415) & normal accident always happen because of failure to develop new paradigms in extremely complex organization. She acknowledges that NASA is a High Reliability Organization (p. 418) but points out at the same time