I have no idea, but the answer may modify the following paragraph in the Threat article in the wiki.
Quote:
Finally, proximity aggro: Mobs will generate an extremely small amount of threat towards the first person they see. In addition, certain mobs are social, and will assist their friends if they are attacked. If any social monster is pulled near his friends, they will also generate aggro towards you as if you had generated proximity aggro - even if the mobs are not hostile by default.
After reading the
Threat Article and it's cousin
Kenco's research on threat on wow-wiki, I don't think that level has anything to do with aggroing social mobs. Being noticed by a mob puts you into its hate list at a hate rating of zero. However if you have the largest aggro radius, you will be at the top of the list. Since any social mobs that get dragged in by the aggro will notice a low level first due to the radius (again). The lowbie will be at the top of those lists as well. This would explain why they go for the lowest toon first. Please understand that this is just hypothesis on my part. Perhaps a more experienced player can verify my conjecture.
You might also be interested in the following article from wow-wiki:
Aggro Radius Edited, Jun 12th 2008 5:34pm by ohmikeghod