
I think they have teeth like rabbits that need to align properly to wear each other down. If one becomes misaligned or damaged they don't do this and the teeth keep growing until they grow into the mouth or out of it.
It is easy enough to cut a Rabbits teeth with plier type nail clippers, so if 'Maloclusion' is your Cavies problem you should be able to do this once your vet shows you, as I doubt Guinea Pigs teeth are much harder.
It could be a mouth abscess or a foreign body causing the salivation.