Sinking or professionaly called "despawning" happens when a certain mob leaves it's biome or if the sun rises will cause a massive despawning of Hostile mobs. Sometimes a Mob will Despawn for unknow reasons for example (Zombies have a bad habit of over jumping which will cause a 75% chance for that mob to despawn or to continue attacking the player), Aliens are different from other mobs since they only despawn outside of their protective perimeter.