Pump and dump is a scheme that attempts to boost the price of a stock through recommendations based on false, misleading or greatly exaggerated statements. The perpetrators of this scheme, who already have an established position in the company's stock, sell their positions after the hype has led to a higher share price. This practice is illegal based on securities law and can lead to heavy fines.
Who is pumping and dumping bitcoin? There is no central authority for Bitcoin. It was created by Satoshi and work carried on by other developers. And when will the dump happen?
In normal pump and dump schemes the product has no utility and the investors dump it after they have found a few fools to buy their stock. this is not the case with bitcoin.
Currently, we do not have much policies for the cryptocurrency market, this is the good point for the rich and the whale control the market without break the law of stock market. Therefore, almost the dump time are large and strong, ~30-50% per day (pump/dump).
This is why people are really after this market because investors have too much liberty here and they are supposed not to follow any particular government or policies.
You should keep in mind that bitcoin and other digital currencies are in high demand because they offer very much margin to investors regarding the investment. They are free to withdraw their coins at any time. Even if the dumps are longer, no need to get worried.