User blog comment:Darkcloud1111/Good Idea, Bad Idea~Mountain Glenn/@comment-24792226-20141015144238/@comment-4010415-20141016012429

To be honest, the fall might have taken out some of Ruby's aura. She fell from pretty high up directly onto a concrete building, and she put her hand on her head when she sat up. So, she may have hit her head fairly hard on the building, thus leaving a weak spot in her aura. See the little black spot in the beam of light on top of that building? That's Ruby. That's how far she fell.

The White Fang member punched her heard enough to spin her off-balance and knock her down. The other White Fang member may have kicked her in a way where her head was hit hard enough to exploit the fact that her aura recently took a pretty hard hit on her head.

Also, ewww, rewatching it, I just now noticed a weird crunch noise when he kicked her in the head.

Now that I think about it... If semblance uses aura as fuel, then that would also explain why she didn't use her semblance - she just took a pretty damn hard hit to her aura from falling multiple stories onto pure concrete.