The various types of enemy AI are programmed to behave based on pressure, aggression, and composure. They react to your levels of aggression, they use deathtraps, and they'll hide behind cover, waiting for the perfect chance to attack. They constantly swarm around you, using the environment, sniping, and aggressively trying to bring you down. The net result of this constant force is that the player is pressured to move, strafe, and stay in motion, never giving your enemy enough time to get a good angle. In short, some guys will hide, while others will attack, but they're react to your behavior accordingly and use the environment to their advantage while swarming you if they get the chance.
