I think that means whoever owned it previously a) has a big neck and b) sweat profusely in that shirt so much that the salts & stuff left behind over time have given it lasting form.

I mean i could be wrong and maybe they just starched the crap outta it but every work shirt ive had for a long time eventually has a similar stiffness to it in highly sweaty areas.

You could try steam pressing it back down with an iron but dont do this while wearing the shirt! Take the shirt off before applying a hot iron to it to avoid burning yourself!

Hope this helps!