I would estimate this one as more of a cult phenomenon, not the local mainstream Neanderthal culture.

After all, what do hominids do in caves, anyway?

They get away from the crowd and it could be an exclusive location where only the true "cave men" held their business meetings, smoked cigars, and stuff like that.

OTOH it could have just been teenagers trying to be rambunctious without getting caught. I doubt they were allowed to break off stalagmites just anywhere.

Either way a truly subterranean non-mainstream sub-culture intentionally differentiated by its members.