Short sellers by definition sell, so the price goes down when they sell. Everyone uses algos these days. Manipulating the price down for an illquid stock could prove to be expensive as you still have to cover your short position and so you end up pushing price back up. What the short funds ideally want to do is to take a position, then bad mouth the company on social media etc and get other traders to push the price down even further for them and then they cover for a nice profit.