My son randomly started "mouthing" and chewing on things

My son (age 5, level 2, preverbal) recently started mouthing and chewing on things, shortly after starting ABA. They gave him "chewies" to keep him from chewing on things he shouldn't, but why did this randomly start? Could it be because he has a little sister who's teething? Did your LO ever start this at an older age and then grow out of it kind of naturally?

TIA :)