The exact mechanism by which secretin is released is still not known.
The mechanism by which acid brings about secretin release is not fully understood but probably involves acid sensing ion channels of the Trp family present in the brush border of S-cells.
Physiological reason for susceptibility to acid: It is because this hormone acts on pancreas and increases the secretion of bicarbonates to increase the pH. It also reduces gastric motility thereby allowing longer digestion within the stomach. So, it effectively prevents deterioration of duodenal mucosa and helps in efficient absorption of lipids by increasing ph in duodenum which is required for better action of lipases. So, its secretion is induced by increased acidity from chyme (so that it can increase pH).
Thus, we can conclude that secretin is acid sensitive.