Killing zombie villager aggravates zombie pigmen

During the night a zombie villager went into the nether through a nether portal i had created. when i went into the nether, there was the zombie, so I killed it. After killing it I was immediately swarmed by zombie pigmen.The aggravated pigmen acted as aggravated pigmen should and no other pigmen out of radius were aggravated. They just acted as if one of their kind had been killed.

The zombie villager was killed on a cobblestone floor in a two by two cobblestone passageway. It would have been impossible for me to have accidentally hit another zombie pigman while fighting him.