Zombie Pigman speed

When I went into the nether to farm some Gold Nuggets, I hit a Zombie Pigman with my bow and then they all came at me, as usual. But in this snapshot, they all came REALLY fast, as if they had a Potion of Swiftness applied to them.

I have not experienced this with any other mob. I don't have any of the new commands running, so I don't think that is the issue.