Why did Zombie Pigmen attack me?

I was playing Minecraft today, and I decided to venture into the Nether to gather some Soul Sand. I walked by a group of Zombie Pigmen, and they started to chase and attack me. This confused me because they are usually neutral unless attacked, and I did not attack any Pigmen after entering the Nether. Why did they attack me if I did not attack them first?

Might of been a glitch or maybee you attacked them before in the past.