When you have for example one angry zombie pigmen and other ones that are not angry around you and reopen the world, all zombie pigmen around you are angry. This probably affects loading an unloaded chunk as well.
The reason for this is that once the NBT data of a zombie pigmen is loaded and the value of the HurtBy tag is not an empty String, it adds new AI tasks. However these AI tasks have no way to tell whether the pigman was just summoned or if it was loaded only so they will start and make nearby pigmen angry as well.
- Summon a pigman
- Switch to Survival mode
- Punch the pigman
- Summon more pigmen
They should not be angry
- Reopen the world
All the zombie pigmen around you are angry