In the Tunnel with the Ettercap hunters/Shamans variant- they were spawning in a strange groups.
It seemed like spawns would detect combat from a good distance away, and even if they were not visible, come charging in to aid a different spawn group.
This led to 4-6 Ettercap hunters at one time, as well as 2+ shamans- and leaving stretches of otherwise unoccupied spider territory. So you'd have a few large fights rather than a bunch of small ones.
As well, the spawn tables for a small group seemed odd, and Underdark giant Spiders were dropping at random intervals. I genuinely thought we were being spiced, then I noticed no DM was online.