The researchers have so far found no remains of early humans, stone tools or other signs of occupation, but they think that Neanderthals made the structures, because no other hominins are known in western Europe at that time. “So far, it’s difficult to imagine that it’s not human made, and I don’t imagine any natural agent creating something like that,” Hublin agrees.
This is pretty much the same argument as "We don't know what's occluding that star, so it must be ALIENS."[0], or most of the speculation around sailing stones[1]. If you don't know what made them, and there's no evidence (other than that they exist), then you cannot say what caused them.
[0] http://www.bbc.co.uk/newsbeat/article/34926314/nasa-may-have...