This is an attempt to fool the Bulls, so traders who believe prices are about to drop. Bear trap is started by buying a large amount of certain coin/token, causing it's price to spike as others start to buy the coins too, in order to not miss the opportunity.
Once the price has been "manipulated" enough, the bull trap initiators start to sells the coins again, but at a higher price than they bought them at. This causes the price to start fall again.