Although the cabin is pressurized, it's still at less pressure when at altitude than ground level. Thus, the more dense "ground" air in the sealed bag expands when up in the air.

An example of the reverse of this is to drink a bottle of water at altitude, and then close the lid really tight, and put it in your seatback. As the plane descends, you'll see the less dense air in the bottle contract, and the bottle itself will "crush".

There are two reasons that your bags of peanuts or pretzels that the airline gives out don't explode when you open them. First, a typical bag of store-bought potato chips is inflated with extra air, so it looks bigger and the chips don't get crushed as easily. The bags of airline snacks are not. Second, the bags the airline hands out are so miniscule, there's little chance of anything but a small "pop" when the pressure inside and outside the bag equalizes.

When I went to the Cape Cod potato chip factory a few years ago, they asked if I would be travelling on a plane. They gave me paper seals for the bags and told me to prick them with a pin before flying and seal them after landing, for exactly this reason.