When an organization or community is doing poorly enough that its smartest people, seeing the writing on the wall, leave for better opportunities elsewhere. This is common in business and other organizations that perpetuate a common goal or mission and once purged it typically accelerates the demise of the organiztion.
For example: Facebook's poor stock performance since it's IPO and lack of favorable stock options for employees puts them at risk of brain drain. Many will likely head over to Google or Apple, but they'll be in trouble.
Mark Zuckerberg: Sheryl, can find me those repots on Zynga's...Hey?! Why are wearing a Google sweatshirt?!
Sheryl Sandberg: Sorry Mark. We had a good run, but your lack of focus on ad revenue and overvalued IPO has caused this drop in FB stock value with no end in sight. I and the others are leaving before the stock tanks.
Mark Z: The others? They're all leaving Facebook? You can't! We'll never survive this brain drain!
Sheryl: Yeah, sorry. You should've thought about that before going public.