Answer: It's likely that she has what’s called "urinary sphincter incompetence". The sphincter controls the outward flow of urine from the bladder. When the sphincter is not functioning correctly, leakage occurs.
This is not an uncommon problem in older female dogs and is due to many compounding issues, such as the position of the bladder and obesity, as well as hormonal changes. While this is a problem of old age, once diagnosed, it can be controlled with medication.