Pippa urgently needs to poo. But she's wearing her white trousers, and if she poops her panties it's going to be very very obvious!
There's no alternative, so Pippa decides this is exactly what she's going to have to do. As her accident progresses, we see her white pants developing a big brown mess. A look inside her panties confirms it's a huge load, but even from the outside of her pants it's about as blatant as it can be.