This used to happen to an ex of mine, once she went past a certain point of desperation she would just start slowly leaking and any sudden movement or pressure on her bladder made it worse. I saw her do something similar to this once or twice when it basically all fell out of her as she pulled her pants down and her body finally relaxed
Wow defo made my dick go hard! She must've been trying so hard to hold it in the car and before she could drop her knickers looks like she already started peeing. You can see the dark patch in her pink panties. Awesome!!