Shorting a currency does not always result in a drop in it's price, if it did everyone would be shorting and getting profit off their investments. I believe short term traders predict a possible drop in the price and take advantage of it rather than causing the drop by selling their holdings.
I am not an expert trader and do not have deep knowledge about the trades and how actions affect the market.
mostly they are those who watch the market all day long that they were able to see which coin have enough potential. This time not all crypto increases some keep on dips that why short selling is quite risky.