Alcohol doesn't cause urinary incontinence in the first place. However, if a person has an overactive bladder or a predisposition to incontinence, drinking alcohol can cause the problem to flare up.
This can lead to urine leakage and bedwetting. Alcohol can act as a diuretic, which means that it stimulates urine production. Your over stimulated bladder will make you need to urinate far more often than usual; for people with incontinence paired with other physical issues, they may not be able to make it to the toilet in time. Alcohol also relaxes the muscles around the bladder which can make it harder to hold in your urine.
If the bladder is weak, this increased urine flow results in more leaks. If you have urinary incontinence, it is recommended to cut down on alcohol and drinks containing caffeine such as tea, coffee and cola.