I had a 5 year old male (Keno) that started leaking suddenly just like you've described. Urinalysis showed no sign of UTI. My vet is also a chiropractor and adjusted him the same day. Leaking completely stopped that day. He said that urinary incontinence can result when the bladder sphincter is innervated at the lower lumbar vertebra. He said it sometimes takes more than one adjustment, but Keno completely stopped leaking with just the one.
A couple of months later I took him to playgroup and he ran like a madman and the next day was leaking again. Took him that day for an adjustment and he stopped leaking that day and never leaked again.
Just something to consider.
ETA
It's mentioned here near the bottom of the list:
http://www.naturallybalancedvet.com/does-my-dog-or-cat-need-to-be-adjusted.html