getting really excited ex. getting a letter from Hogwarts at the age of 13. Or getting really scared really fast ex. HONEY IM PREGNANT... Really its just a matter of your bodys muscles getting tension really fast or relaxing really fast..
Also aging, which isn't really sudden, but might seem sudden the first time one actually loses control and has an accident.
Bladder infection or UTI that's untreated, sneezing, coughing, vomitting... is it just a one-time thing?
Pregnancy, too, I've heard, though it's never happened to me when I've been pregnant.
Lol nice, now u get some pts for that :-)