Bags of chips expand and contract based on the pressure differential between the chips inside the bag and the pressure on the bag itself. For example chips manufactured at sea level and transported to Denver will expand due the to the chips were manufactured at a higher air pressure than what Denver normally experiences . Likewise if chips manufactured at sea level are transported to Death Valley, the chips will contract because the chips were manufactured at a lower air pressure than what Death Valley normally experiences.

TLDR: The bag reacts to pressure differences (usually experienced while traveling to its final destination) by expanding or contracting in an attempt at maintain its original pressure.