For the curious, they're terrified of Yuuka Kazami. It's from a series of comics (not quite 4koma) on the Pixiv Fanbox page in which various Touhou characters get comically large bumps on their heads (like spheres as large as their heads, on top of their heads). The setup is that Cirno's decided to steal some pastry from Yuuka: Drop a large ice chunk on her head to knock her out, snatch the snacks, and scarper.
