If you were a pre-IPO stockholder you would also be a post-IPO stockholder. Selling some of your shares in the IPO at $20 and the rest in a few months at $40 may better than selling a few at the IPO at $40 and the rest in a few months at $20.
If their intent is to cash out now, they are probably happy enough. Anyone with a long-term interest in the company would probably prefer for more of that valuation flow into the company's coffers, rather than to the underwriters and others with little long-term interest.
What are you talking about? The "bump" off of the fake price that got made up yesterday?
Dropbox was worth $12b yesterday, just like it is today. And a few banks suckered Drew into screwing himself, his investors and his employees by selling shares at a $8b valuation.