What are causes of tongue skin tags?

Trauma. What you are describing is a fibro-epithelial polyp. These are thought to be caused by minor trauma from cheek or lip biting. People of varying ages can be affected by these polyps. Treatment is surgical removal. However, it is best to have them checked and possibly biopsied by an oral surgeon or dermatologist.
May be normal. Normal anatomy of the tongue may have tags of different sizes. Food, smoking, dipping, tongue splinting are possibly sources even cancer. See a dentist or physician for diagnosis.