The most common causes of acidity are:

Consuming acidic, spicy, or fatty foods in excess: Tea, coffee, carbonated beverages, soft drinks, excessively spicy meals, and fatty foods like pizza and doughnuts can trigger acidity.

Irregular eating habits: Eating irregularly, skipping meals, or eating just before bed can cause acidity.

Obesity: Being overweight or obese increases pressure on the stomach and can cause acid reflux.

Stress and anxiety: High levels of stress and anxiety can stimulate excess stomach acid production.

Smoking and alcohol consumption: Smoking weakens the lower esophageal sphincter, allowing stomach acid to flow back into the esophagus. Alcohol can also irritate the digestive system.

Certain medications: Aspirin, ibuprofen, and other NSAIDs can irritate the stomach lining and cause acidity.

Hiatal hernia: A condition where part of the stomach protrudes through an opening in the diaphragm, weakening the lower esophageal sphincter.

Pregnancy: Increased pressure on the stomach due to the growing fetus can cause acid reflux.

Lying down immediately after eating: This allows stomach acid to flow back into the esophagus.

Certain medical conditions: Zollinger-Ellison syndrome, diabetes, asthma, and connective tissue disorders can increase the risk of acidity.

Citations:
[1] https://byjus.com/biology/acidity-symptoms/
[2] https://www.lybrate.com/topic/acidity
[3] https://www.starhealth.in/blog/acidity-causes-symptoms-risk-factors-and-home-remedies
[4] https://www.1mg.com/diseases/acidity-42
[5] https://dpuhospital.com/blog/what-are-the-causes-of-acidity/