They are contained within a small cave and increases the change you may be attacked by several at once. When multiple attack at once it is likely to take heavy damage in a short space of time. It is best to not use abilities that target multiple enemies unless you are prepared for attacks from multiple Chaos giants. Often when entering the cave one giant will attack you, after defeating this giant another will often start attacking, if you do not wait long enough for the second giant to attack and click on one yourself this will often lead to multiple giants attacking. If this happens the easiest way to stop it is to leave the cave for about 30 seconds and then reenter. Upon reentry usually only one giant will begin attacking again. Another option is to try to position yourself behind the crystals in the cave to try and block most of the giants while you concentrate on the one you're attacking.
The chance of receiving better drops is increased by the Ring of fortune, Enhanced luck potion, Collector's insignia (charged), or Luck of the dwarves.