Aggro reset

Aggro reset refers to a mechanic when a mob or boss clears the accumulated threat of characters engaged in combat with it. The mob or boss will then proceed to attack the next player that causes the most threat, until the tank does more threat either from damage or taunting. Note that a complete aggro reset of an NPC is not synonymous with Threat Reduction.

Causes
There are several reasons that a hostile NPC may reset aggro:
 * An encounter mechanic in a server-side script, particularly:
 * At a stage transition including swaps with another boss NPC like in the Twin Emperors encounter in the Temple of Ahn'Qiraj.
 * After a major ability like a prolonged AoE or Cleave.
 * A bug in such a script may cause unintentional aggro behavior including a reset.
 * When the target character:
 * ...exceeded the NPC's maximum pursuit distance, sometimes called chaining.
 * ...engaged or moved to a location that the NPC cannot reach when it is only capable of melee attacks.

Considerations

 * Scripted aggro resets that are a component of a boss encounter should prompt players in a DPS role to reduce their damage until the tank has had time to rebuild a significant amount of threat.
 * Players may use a raid addon such as Deadly Boss Mods to ensure that they're prepared to cease or reduce damage when an aggro reset is about to occur if it is a predictable event.
 * The next target after an aggro reset may not be based on threat. Again, see Twin Emperors.