You aren't going to find any legitimate research that caffeine causes adrenal fatigue or is associated with adrenal fatigue because there's no such thing as adrenal fatigue.
True on "adrenal" fatigue, but it does increase cortisol levels and blood pressure. Chronic use will also lead to increased run-of-the-mill fatigue.
Caffeine works by blocking adenosine receptors, which is part of the mechanism for feeling tired. So the brain compensates by creating more receptors and flooding the brain with adenosine.
If there are no caffeine molecules in the brain to block the receptors, you will crash. Consume more caffeine, and voila: you feel normal again.
