Life is a stormy river

Physics™

But seriously it's because she manipulates her body in order to keep her hips and torso on top of the bridge. She's not just ragdolling. The bridge kinda pushes her back toward the middle too.

It's like a Japanese show that was shown on TV. This show had a similar challenge. The player stepped onto a rickety rope bridge, but was prevented from reaching the end by two guys standing on the ground below, who were shooting soccer balls at him. The bridge swayed a lot, it was difficult to stand and there were many funny moments. A safety net was stretched under the bridge.