It is not often that I agree with the FBI, particularly after Director James Comey's fumbling Congressional testimony on why his agency should have access to all encrypted communications. However, his response to three major computer outages (United Airlines, the New York Stock Exchange, and The Wall Street Journal) taking place all on the same day was the right one. Those who investigate criminal acts cannot afford to be "big believers in coincidence."
Indeed, this may be one of those cases in which investigators have to exercise the full scope of imagination to come up with a wide variety of qualitatively different hypotheses. One should be able then to assess systematically each hypothesis. Some will be easy to eliminate, while others will require more intense scrutiny and investigation. (The very question, "What will it take to refute this hypothesis?" can be a valuable driver for deciding the directions investigation should take.)
Even the hypothesis that this was, indeed, a confluence of accidents needs to compete with the other hypotheses. The point is that confluence is not necessarily a matter of coincidence. In this case, for example, it may simply be the result of the general decline in software quality, regardless of the particularly function being served by the software. In other words things went south for United Airlines, the New York Stock Exchange, and The Wall Street Journal for the same reason that they keep going south for OS X with increasing frequency.