Why do zombie pigmen remember you?



Okay, so I've been in the nether just now, and the time before that was a looong time ago. When I went through the portal and out into the Nether, zombie pigmen sprung up behind me and immediately killed me! I used /back, and they went at me again! I collected my stuff and when the pigmen pushed me back I fell into lava, losing my diamond sword and leather armor set (which isn't a big deal I guess) but it's still annoying to find they remember you after a long time and attack you. So if you had a full set of fresh diamond armor it would kind of bug you to find a horde of pigmen ready to strike and you lose it all. Can someone pleeeease tell me why this happens??? :zombie: :pig: :(