Wine legs are the droplets of wine formed inside of a glass. As alcohol evaporates,it pushes up wine on the inner surface of glass forming an upside down crown like structure.
Tears of wine or wine legs indicate the alcohol level in wine . Higher the alcohol better Tears of wine. It does not indicates the quality of wine. wine legs may appear in sweet wines also, because of viscosity of wine.
It is a scientific phenomenon. it never happens in closed bottle, because there is no evaporation of alcohol. evaporation is important to form wine legs.