How does the citric acid in citron potentially cause heartburn or acid reflux?

It can relax the lower esophageal sphincter (LES)

Heartburn and acid reflux are gastrointestinal complaints frequently associated with high-acid fruits like citron because of their citric acid content. The mechanism involves the relaxation of the lower esophageal sphincter (LES), which is the muscular valve situated between the esophagus and the stomach. Normally, the LES remains tightly closed to prevent stomach contents, including corrosive acid, from ascending back into the esophagus. When this muscle relaxes due to the influence of the acid, stomach contents reflux upward, resulting in the characteristic burning sensation felt in the chest region. For individuals already managing gastroesophageal reflux disease (GERD), this effect can acutely worsen their underlying condition.