Skin tags are very common and generally occur after midlife. They are said to be caused by bunches of collagen and blood vessels which are trapped inside thicker bits of skin. They are believed to be the result of skin rubbing against skin.That is why they are generally found in skin creases and folds.kin tags can appear on any part of the surface of the body (skin), but most typically exist in areas where skin may rub against skin, such as the: 1  Eyelids. 2  Axillae (armpits). 3  Under the breasts. 4  Groin.