1) Traders were believing, briefly, that going from Grey to Pink would cause a rally. This is NOT the case for the vast majority, and certainly hasn't happened lately to the many ones that were recently move to Pink.
2) Companies / traders with worthless unsalable shares suddenly saw an opportunity to unload! They paid IR folks to push it and manipulated the trading to unload. Notice how quickly virtually every one of them retraced? Notice how low the volume was for the few that popped? Hardly enough trading to make a buck...
And no evidence whatsoever that short covering played a role!
And so far, not a single "Experts Only" exchange stock has moved to Pink!