Most investors especially those who are known to be influential on so media or well known investors are always in found of doing this to increase their profitability towards their holdings, and now the reason for this is to attracts more investors it may be their followers and when they noticed they always has lots of people who keep buying whatever token/coin they tweeted you would see them go buy a coin and tweet again along the line when people keep buying the token you would see that prices increases and after which they sells off and take profits leaving others with empty worthless token/coin.