Women wear thumb rings for a variety of reasons but usually simply for fashion. One historical reason for a woman to wear a thumb ring was when her husband either died or went off to war. The woman wore her husband's wedding band on her thumb since tat is the only finger on which a ring made for a man is likely to fit.
Thumb rings as a fashion trended started in the 90s when popular actress Julia Roberts wore one in a movie called "Dying Young." Later in that decade, the rumor got started that women wore thumb rings, particularly on their right thumbs, to denote that they were lesbian. This seemed to be reinforced by lesbian singer Melissa Etheridge wearing a thumb ring in this fashion.
Generally, however, women wear thumb rings solely for fashion. Thumbs historically represent willpower and independence, so one reason for a woman to wear a ring on her thumb is to demonstrate a non-conformist attitude or to demonstrate that she is confident. Palmistry traditions say that a thumb ring can focus positive energy and increase assertiveness. This also has much to do with what the thumb historically represents in terms of willpower and pride. Another reason for a woman to wear a thumb ring is for the simple reason that the ring does not fit on her other fingers.