The specter of rising interest rates in the US (driven by higher inflation expectations) appears to be a factor.[a]
Fast-growing companies that are investing aggressively today and whose profits lie far in the future, in particular, are exposed to rising interest rates, due to the higher duration of such companies' cash flows.
Duration, for those here who don't know, is a measure of the sensitivity of present value to interest rates.[b] Duration rises with the amount of time an investor must wait for cash flows, and vice versa.
For example, the present value of $100 of cash flow in 10 years, if the 10-year rate is 2%, is equal to $100/(1.02^10) = $82; if the 10-year rate rises, say, from 2% to 3%, the present value declines to $100/(1.03^10) = $74, or a -10% decline. However, if the $100 in cash flow is in 30 years, and the 30-year rate rises from 2% to 3%, the present value declines from $100/(1.02^30) = $55 to $100/(1.03^30) = $41, or a -25% decline.
In this example, an increase in duration from 10 to 30 years caused the sensitivity of present value to a 1% rise in interest rates to change from a -10% decline to a -25% decline. The longer an investor has to wait for cash flows, the greater the sensitivity of present value to changes in interest rates.
The same ruthless logic applies to companies. The present value of companies whose profitability is in a distant future declines much faster when interest rates rise than the present value of companies certain to generate cash flows in the near future.
Until recently, due to historically low interest rates and no prospects for inflation, the stock market has been rewarding high-investment companies that are sacrificing current profits for growth.
If interest rates continue to rise (along with expected inflation), I'd expect this abruptly to change -- in which case, strap on your seat belts!
[a] https://www.ft.com/content/af1f8e4a-0a23-11e8-8eb7-42f857ea9...
[b] https://www.investopedia.com/terms/d/duration.asp