Tarnish (AKA toning) is caused by exposure to air, chemicals, and/or dirt & oils from human fingers. A chemical reaction causes oxidation of the metal and the change in color. The same thing...
Oxidation and corrosion tarnish pennies
darker soft drink will remove the tarnish form the pennies
Damp conditions tarnish copper and bronze pennies quickly. Bleach can also tarnish them.
With organic matter from fingers (skin oil) sneezing, etc.