A bear raid is an attempt by traders to drive down price of a security by heavy short selling. The selling may be accompanied by the spreading of (negative) rumours.
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.