Why does my cat freak pout when we try to...?

He has a couple little clumps of matted hair on his rear end (Like since when has my baby not been brushed...?) ANd when I tried to cut it off, he freaked out yowled and clawed and stuff...? He has been neutered and we are pretty sure its not his... You know... Or anything but a piece of matted fur.. WHy is he freaking out?