Why Does My Dog Urinate When Left Alone?

Most likely this is a nervous reaction from your dog known as 'separation anxiety'.  If your dog is not crate/kennel trained, I would recommend doing so.  There are many reasons why a kennel is a good thing for your dog, but mostly, dogs are animals that, when wild, sleep in dens.  The smaller area of the kennel is actually very comforting to them, once they accept the kennel as their 'den'.  Also, dogs naturally do not want to go potty in their den so the urinating should end if the dog is kenneled when you are away.  To help the anxiety when you are away (especially at the beginning of crate training), place a clock that makes an audible 'tick' noise in the kennel wrapped in towels with your dog (or on the outside of the kennel if you are afraid of their chewing the clock) as the ticking mimics the heartbeat of the mother dog (if your dog is still a puppy) or your heart beat if the dog likes to cuddle with you.