I feel like they're shocked at what a character we can't see (and who I also feel is meant to be the viewer cause they kinda seem like they're looking at you) is doing.
Azuretan said: Interesting that the artist put Creative Commons on the picture.
Well, how about that? It's on post #750537, too (but none of the artist's other works on Danbooru). It's even by-nc-sa, making it one of the rare works that actually has implicit author permission to be reproduced on Danbooru.