Semil Shah sent out this tweet with this image below that was put together by Tomasz Tunguz. There are a few things causing this meteoritic rise of growth rounds from my perspective.
- There is a lot (and too much) private capital available for startups (private companies) right now. This creates an opportunity to raise large rounds.
- Due to cross-over investors participating in the private markets, some of these startups don’t have to go public to raise money. IPOs are partially used to create liquidity for investors (and employees) who participated in private rounds. Many of these large rounds are now providing the same liquidity that IPOs used to create.
- Some might argue that the public markets are partly broken, which is driving the large market rounds. Not sure I agree with this perspective though.
Shai's Blog 
I agree with your view on point #3. I don’t think public markets are broken but maybe going public isn’t actually what is best for the company? We’ve seen it time and time again, reporting to shareholders, quarterly can cripple companies ability to think long-term.