Is Yukari rushing to use the bathroom? What I can decipher of the commentary doesn't seem to explain what is going on.
Through the series Yukari infiltrates different schools to learn about their tankery and general status to try and get a small upper hand in battle. When she does so, she records it in a movie as if she was doing some sort of documentary.In order to disguise herself as someone from another school, she heads to a bathroom to change her uniform. In the main series, the first time she does so, she forgot the camera was recording and almost immortalises something memorable like what you can see in the picture.
I definitely remember the spying part, as it is brought up in a lot of the ancillary materials, but evidently I forgot about the video recording of herself part.