It's always in the best interest of the people who benefit from a bubble to make such driving factors less visible, so the bubble can continue longer - and hindsight is, as they say, 20/20.
Which is to say... just because you can't see the causes yet, doesn't mean that they're not there. The Fed is happily handing out cash and people are borrowing large amounts of it to shuffle around while they can. There are lots of areas where people are making what normally would be considered risky bets which keep paying off because the economy is strong, and they just leverage themselves in the extreme to take advantage of that.
So whether it's another real estate collapse or another SV-funded startup collapse or some other kind of collapse that isn't yet apparent, after the other shoe does finally drop we'll be kicking ourselves for not noticing how bad things had gotten on the way there.