You need to get familiar to the words pump and dump. Pump and dumping basically caused change in supply and demand. Pump means the stage of keeping on buying, where dumping means the stage of "dumping" that is selling your coins. Pump decreases supply and dumping increases supply which caused the price to rise and fall. Investors dump when you want to exit with their profit. The price falls down, and new investors come and starts pumping the price.