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?