How do people usually get adrenal fatigue?

Does not exist . Despite the popular belief that it does, helped by numerous self-help books and websites, there is no disease entity called "adrenal fatigue". The Endocrine Society has a statement on it and the Hormone Foundation has information at www.hormone.org. Adrenal insufficiency is a real disorder and can be life threatening if not treated. In the US, it is most commonly autoimmune in nature.
Multifactorial. Adrenal fatigue is a multifactorial consequence of hormonal, immune system and nervous system dysfunction resulting from genetic predispositions and dietary and environmental insults. These factors stress your adrenal glands beyond their ability to respond and dysfunction occurs poor diet, suboptimal sleep, food and other allergies contribute.
Multifactorial. Adrenal fatigue is a multifactorial consequence of hormonal, immune system and nervous system dysfunction resulting from genetic predispositions and dietary and environmental insults. These factors stress your adrenal glands beyond their ability to respond and dysfunction occurs poor diet, suboptimal sleep, food and other allergies contribute.
Gut. Gut dysbiosis - bad bacteria prevents protein digestion creating lack of tyrosine and adrenal fatigue, the other things adrenals need 6 hours of sleep, minerals including magnesium, vitamin b, salt, water and stress all these depletes adrenals.