Why do guinea pigs bite their cages?

That can be a sign of boredom, just a stereotypical behavior. Make sure that they are getting out, getting their exercises, that they're stimulated enough, and they have enough enrichment. You want to be careful if they're biting their cage because, many times, cages are metal, and they can chip their teeth, so eliminate that as much as possible. Find toys or something else to distract them. You can hide their pellets around their enclosure, so they have to hunt for them.