You are not alone in this. It is natural cat behaviour to be nosey at, peruse, sit in, tail chase, or rub noses with any box, bag or other receptacle left within paw reach.
They do this because even the most seemingly sensible and intelligent cat will think there’s something to chase. In a big cardboard box they will poke at the corners or chase their tail. In a smaller bag they will sit and await the return of the beastie that lives there. It’s an unshakeable conviction that somewhere in the vicinity of this bag is prey. And the root of this conviction? Hallucinogenics.