Is this the normal step prior to any bubble?
The euphoria of making quick bucks over the investment on a unicorn may be the first step, then, watch carefully for their transition from an idea to a concrete evidence that they can deliver the service for what they were created in the first place. But, if they can't deliver? What's next?
Most, if not for a few of them unicorns, they expect to be bought out by a big shark like Google or Facebook. If nobody then is picked, they will not be worth anything if they can't survive by themselves.
Otherwise, the answer would be the famous "it's the economy idiot!".