Why does incontinence sometimes happen with cystitis?

Inflammation: The inflammation associated with the cystitis can often cause irritability of the bladder muscle resulting in bladder spams. These spasm can often be associated with urine leaking.

Bladder irritation: When you have a urinary tract infection it irritates the bladder. This makes you go more often (frequency) and can make it burn when you go. It can also cause you to have to go really bad (urgency). So bad in some cases that it is hard to make it to the bathroom on time causing you to be incontinent.