It was level related, every mob that is level 25 and higher would always aggro you (proximity aggro only, range rather small, see below on levels and aggro range) even if it's green and not just when you're sitting. Of course all undead would aggro you but also would aggro anyone they passed over and act like they're rooted. So if you train a skeleton / zombie or whatever past someone it would start attacking that person and pursue it, always attacking the nearest target and they were highly aggro and jump a passive bystander. This certainly poses a threat to be used in grieving as you could gather up a load of undead and dump them off on someone afk / lower / about to die, but that's how it was, you could dump your undead train on others and let them deal with it or use them as a stepping stone to survival by sacrificing them.
In general I think aggro ranges on p99 are in some cases too small, I feel like level 59+ when pulling some camps, assist range of mobs would decrease as you gained levels so a level 1 wandering around gfay would be jumped by an orc centurion over 100 range away (spell cast range / throw range) while as you closed in and surpassed them in level the range grew smaller to the point you could single pull them if they stood fairly close together. A good example would be Sebilis necrosis camp where at 59 you could single pull most of the double krup spawns and most of the other upper level mobs (no idea if that is how it is here). Currently a lot of mobs can be walked past at fairly close range which doesn't feel natural and in a lot of cases makes travel and hunting rather easy.
A bit more recent post (2005) but it talks about frenzy and reaction radius of mobs:
http://forums.station.sony.com/eq/po...topic_id=32479