Humans cause acid rain mainly by burning fossil fuels. Coal-fired power plants, factories, and vehicles release sulfur dioxide and nitrogen oxides, which react with water in the atmosphere to form sulfuric and nitric acids that fall as rain, snow, or fog with a pH typically around 4 to 5.
Volcanoes and lightning add some of these gases naturally, but human emissions are the main driver. Pollution controls such as scrubbers and cleaner fuels have sharply reduced acid rain in North America and Europe since the 1980s.
Source: Acid rain, Wikipedia
Image: Origins of acid rain, public domain, via Wikimedia Commons.