Board Thread:Speculation House/@comment-4830106-20131108040231/@comment-14138255-20140323175709

Perhaps using the Semblance redirects the full Aura to a specific point to perform a certain task, thus Weiss while using her Semblance was also left vulnerable to an attack she did not anticipate. This holds (or maybe doesn't) water with the fact that none of the others were harmed while using their Semblance and the fact that Weiss' combat style leaves her Aura constantly overtaxed.