I tend to perceive what's behind it, in a general sense, is a closed mind. The reason they are so damn sure of everything they are doing, is because they have formed their conclusion based on, as Trump is quoted as saying in the article, "what they already know" and have long since closed the door on learning anything more. To protect their incompetence driven fragile egos, they will firmly stand behind their knowledge to the detriment of all logic, reason, data, or conflicting experience. Yet, anyone with an open mind knows that "what one knows" rarely holds absolutely true for more than a fleeting moment in time. Therefore, those with more open minds allow themselves to question, to try and better understand by incorporating additional information into their historical knowledge and adapting their comprehension accordingly.

Anatomy of a Trump rally: 76 percent of claims are false, misleading or lacking evidence

[washingtonpost.com]

It has been my observation that persons with low I.Q.s, 100 or less, seldom exhibit the unwarranted confidence as described in the Dunning-Kruger Effect. It is those with 105 to 120 that I have observed to be the worst offenders.