Yes, so basically picture a cave like a giant jawbreaker. When a jawbreaker is left in the sun (and yes, Mythbusters proved this), the different layers of candy expand at different rates. This causes pressure buildup and eventually explosion.

A cave isn’t all just one solid type of mass, it’s not all a single boulder like many people depict due to the limited coloring of old cartoons. You’ll have many different forms of sedimentary, igneous, and metamorphic rock all around the cave. And a fire is hotter than depicted on TV, that’s the whole point of a campfire (a part of why the original question in the OP intrigued me). So the simple act of starting a fire can cause a chain reaction which destabilizes a cave section and causes a collapse. And with caves being as intriguing as they are, you don’t want to ruin humanity’s chances of finding a cave section.

People upvoting a lot of the confusion-based replies shows both sides here have things they never expected they didn’t know (while downvoting my own confusion-based replies, for whatever weird reason).