What causes incontinence in alzheimer's?

Few causes. Incontinence can be caused by a bladder infection, and over active bladder ( bladder spasms) or in an alzheimer's patient sometimes they just get distracted and confused and don't remember to use the bathroom. Follow up with a gynecologist, urogynecologist or urologist.
It is functional. Usually it is because the person becomes too confused to know they need to use the toilet or too confused to find the bathroom.
Ad. Sensing need to void urine requires a sense of response to internal stimuli, often lost in ad. Then there is the need for quick response. As well as progressive overactivity of bladder detrusor muscle inappropriately.
Brain Cell Loss. Alzheimer's disease & other neurodegenerative disorders (such as parkinson's) are ultimately devestating because of the loss of brain cells. Eventually, the cells that control the bowels & bladder are effeted & the result is incontinence. That being said, there are certainly things that can be done to help manage like frequent reminders to go to the bathroom & limit fluids in the late evening.