Why did my Zombie Pigmen form a flash mob?

I was journeying to the Nether when I noticed the most curious site outside my portal enclosure:

The mob started with about six, and has over doubled in numbers over the last half an hour or so. Every zombie pigman in sight has congregated in this one spot, and more continue to come. They just huddle together, jumping constantly. Occasionally one will stray from the mob, observe me or a mushroom or something, then return once more. Neither I nor anyone on the server has aggravated them.

I've checked the wiki, which confirms that they do tend to group together, but says nothing about spontaneous dance parties. What causes this behavior, and has anyone seen something similar? Is this a known pastime for zombie pigmen, or simply a pathing glitch in the nethers of SMP?

Zombie Pigmen tend to travel in groups. That's why they are so hard to kill.

This is entirely speculation, but I believe that there are two possibilities. One thing that might have happened is that they spawned in about three groups of four each, then banded together to create a large group. According to the wiki: Zombie Pigmen spawn in groups of 4, no more, no less. However, groups can merge and form an even bigger group. Also, are you sure that nobody has provoked them so far? It's not that hard to smack one :).