I've had three guinea pigs before and some will not stop chewing until the item is completely gone! Their behavior might have changed if you are spending less time with them or if there are more sounds around. Animals are finicky, so they could simply be bored/restless and looking for something to do.
Here's my advice...
- Don't give them cardboard (unattended) with tape or anything printed on it, because even if they don't eat it, the adhesive from the tape probably isn't safe to chew on. Just because they can process some paper, doesn't mean they should. It is fun and cheap to make tunnels out of boxes, just make sure it's no more than a little corner-nibbling! :)
- Guinea pigs don't live in the wild, but their ancestors were eaten by hawks and other large birds, so they are natural burrowers. Because of this, they should always have have access to a hiding place of some sort. (Though you can coax them out with food or occasionally play with them without the house.)
- I'd recommend getting a non-dyed pine hideaway or one of the "chewable logs." These are tougher for them to eat and are safer for them.
Also, like Michelle suggested, chew sticks are great! Be careful not to get any toys that they can get "stuck" in (i.e. large loops they can get stuck around their necks). - The chew sticks are always a safe bet!
Good luck!
-- Update --
Just like with other animals and humans, there is no nutritional content in paper-based products. So if they fill up their tummies on cardboard, they could be missing out on nutrients from their regular food and hay. Also, too much of a non-food could cause a blockage - see previous link and this one.