Every action by your character generates some degree of threat (a.k.a. hate) which may trigger the attention of mobs that are nearby. Some aggressive mobs have a low trigger threshold and therefore respond more readily than a more passive mob. A mob's aggression (a.k.a. aggro) is targeted at whichever avatar (or pet) is generating the highest threat. This is a major difference between how AI (artificial intelligence) mobs behave and how another human player would behave.
- Exploiting that fundamental cause and effect relationship between threat and aggro is what makes grinding in dungeon parties a little mindless or boring. If the developers ever get the idea to evolve the AI algorithm to also consider which human controlled avatar or pet is the easiest prey to kill off then that might dramatically transform this game. And may kill many healers in no time as well as no pixies around.
Your action may be seemingly harmless and yet it presents a threat in the eyes of nearby mobs. Healing yourself or another player is one example. Using a scroll on another player or pet is yet another. Even sitting/standing still generates threat and may trigger a mob to react.
Some active skills are specifically designed to raise your threat level. These are provocation skills (a.k.a. crowd control skills) and are extremely useful in a dungeon party to steal mob aggro away from a weaker party member.