That is my first thought... a fisher. There aren't too many animals out there that will mess with a porcupine. And if they do, they won't a second time. Except for my German shorthair...I suppose his first encounter and 180 quills in him wasn't enough...he needed a second taste a few months later.
Bobcats will kill a porky just the same as fishers do. They slap them in the face until they die.
I found a pile of fresh coyote scat the other day with nothing but porcupine hair and quills in it. That had to hurt.
A hunter shooting one and leaving it lay is another possibility....but typically there is always a carcass...or pieces...this was just a mess of quills and fur...no telling really...was a big hollow im sure holds cats and fisher...and was on the transition.of the hemlock bottom, hardwood flat, rhododendron river bank...good cat and fisher territory