I want to believe they were hired to be part of a music video/show and had to perform this in front of a camera. It goes viral in their school (think the school spin off show) and they suddenly get a lot of attention and fans. Jean is loving the attention, Marco is flattered but still flustered, Reiner just likes the appreciation and Bertholdt is flustered out of his mind (specially when his crush Annie finds out about the video). Funny scenario aside, they look REALLY good in this music vid. Marco and Bertholdt especially