...?
I don't see it.

Her shoulders I guess?

it isn't her shoulder, she's hunched forward so that looks normal. I don't see any bad anatomy at all.

If that's the case, the artist isn't very good at perspective. It's not easy to tell that she is leaning forward, so instead it looks like her upper body is squished together.

It's not the worst anatomy I've ever seen. Just bad perspective.

I think she's leaning forward while trying to keep the zombie pose intact, hence the stiff shoulders.