When you damage an enderman by using for example a poison splash potion or letting an anvil fall on him, he will stay angry forever unlike the normal endermen who are only for some seconds.
This relates to: MC-65668
Linked issues
Comments 4
The code that handles that is part of the AI update routine, which the NoAI tag specifically disables.
Because that's handled directly in the function that handles attacks and damage. Entities are a huge mess of old and new code that Mojang hasn't gotten around to cleaning up yet. They've slowly been moving behavior from the old system to the new, but a lot of mob-specific behavior is still handled in code that predates the "New AI" system.
Confirmed for 1.8.2-pre6