From the linked article:

When an error occurs, the first thing that needs to be done is fix the error. Minoura recalls that Ohno used to order them to ask the question "Why?" five times over because "that way you'll find the root cause, and if you get rid of that it'll never happen again." However, Minoura emphasizes that on-the-spot observation rather than deduction is the only correct way to answer a "Why?" question. "I'm always struck that the five-why method doesn't seem to be working as well as it should be because there's been a lack of practical training. The reason is that they end up falling back on deduction. Yes, deduction. So when I ask them 'Why?' they reel off five causes as quick as a flash by deduction. Then I ask them five whys again for each of the causes they came up with. The result is that they start falling back on deduction again, and so many causes come back that you end up totally confused as to which of them is important."