I don't use much social media, but Snapchats discovery cards are over run with creators using that face. It's excessively abundant in the channels there.
It feels like a forced attempt at viral marketing. "Maybe if we act shocked, they'll be shocked too!"
My best guess is, that as humans our brains are especially sensitive to the facial expressions of other people. And, a shocked expression is an incredibly important signal. Looking at someone wide-eyed with an open mouth in front of you implies there is something you really need to pay attention to.
Just watch how babies react when you make the same face.
I suspect social media just hijacked and metastasized the response.
It feels like a forced attempt at viral marketing. "Maybe if we act shocked, they'll be shocked too!"