@Tonikill1 It's not "much" it's "often" (so technically it's more likely she urinates too little each time).
As for the causes..
Women have an easier time getting Urinary Tract Infections, which is the most common (relevant) cause for having to pee "too often/too little each time".
Also, since he used to be a boy, she's probably not used to the differences in bladder control, making MC the most likely suspect for failures.
As for the others, they're kids in the first round. Also for women things that can cause unwanted urinations are such as low estrogen (they're hitting puberty) and their diet. (Spices, caffeine, etc.)
And of course, drinking too much because they're having fun at a party. XD
(And of course, because the mangaka thinks that piss is a joke that keeps on giving and can be milked for all it's worth
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)