Neanderthals built cave structures – and no one knows why
nature.com
nature.com
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...