The most common cause and reason for postmenopausal bleeding is due to inflammation and thinning of the lining of the vagina which is also known as atrophic vaginitis.
The atrophic vaginitis is caused by oestrogen levels lowering due to estrogen hormone not being produced anymore after menopause.
However postmenopausal bleeding can also sometimes be caused by cancer but it only happens in about 10 percent of women who experience postmenopausal bleeding but it's a good idea to get to the doctor anyway and get tested to be sure.