Swarm Kiting Mob Aggro question.
 

I was swarming in The Burning Woods, I took the time to pull just about everything I could (pull took over 30 minutes) and I started my swarm. A few minutes in and I think I noticed a bunch of mobs leaving my swarm to walk back. Does anyone know how to counter this behavior or could just explain the mechanics behind it? Thanks to anyone who can impart some knowledge.

leash, have to say X units near the mob

(Assuming you didn't leash them by getting out of range.)

Yes. If you don't do anything to an NPC for long enough (15 minutes?), it will sometimes forget you and leave. This might not be the case for social NPCs, since I think their friends will re-social-aggro them onto you; whereas non-social NPCs might just get bored and wander off. You might have to re-tag them.

To clarify: I am aware of the distance leashing, I didn't have any issues getting them to the kite spot and it was apes, skellies and hornets that all up and left during the kite. 2/3 of those social aggro but still dipped out on me. I guess it's possible that someone else had a pull of their own and zoned while I wasn't looking and it was their mobs walking back but I doubt it.

they'll get bored if you don't generate any aggro after 10 or so minutes. Social mobs won't always social back in so maybe try sitting while running aroudn to force them back in. Skeletons for sure won't social back in; you'll have to tag them back in.