Once the price falls, the traders can cover short positions cheaply.
Once they have built long positions, they then wait for the price to recover and sell at a profit.
A bear raid is usually carried out by a group of traders working together - a single trader is unlikely to be able to sell enough to drive the price down. Even if a single trader could force the price down, the manipulation is likely to be more apparent to regulators who could trace the selling to a single source.
Bear raids are (obviously) a form of market manipulation and illegal in most of the world.