Zombie Pigmen target players who have not provoked them

I was playing on an SMP server in the Nether with a friend. I was on Creative (I think I was invisible), and I attacked a zombie pigman. They weren't provoked.

What I expected to happen was that the Zombie Pigmen would do nothing else, since they did not attack me.

What actually happened was that as my friend approached the zombie pigmen, they attacked and killed her.

Just do what I said to reproduce the bug, but I don't think it might work.

I can't provide a screenshot, sorry.