I think it all started with Facebook. TechCrunch was reporting every 2 weeks on how Facebook's valuation grew another $5 million. What kind of "news" is that, unless you want to profit from the extra hype of Facebook's valuation?!
Many people got very rich from Facebook gaining such high valuation before IPO. And now many other VCs want to replicate that kind of sort of almost ponzi-scheme type of investment.