In my experience this 'residual aggro' is due to traps that have fired, but had their targets run out of range. I presume this leads the trap to remain in existance. As for why traps are considered an aggressive NPC, I do not know. Perhaps it is the way in which they are coded; I remember once missing an 'aedispelltrap' in Velks while shooting arrows in the lower dogs labyrinth.
I will admit this is anecdotal and I've not done any test. The only situation I have to back my theory is a case is one LDoN mission in which someone mentioned they couldn't loot the chest. Being that I wasn't interested in what was in it I was heading back to the zoneout and disarmed a few traps along the way. Without camping or otherwise dropping aggro the person was then able to loot it. Also, there have been instances where I've zoned out on my ranger to run back to attempt to loot a chest. Usually I end up passing some traps that trigger but do not land, and as I now expected, I was unable to loot the chest.
Thin, I know, but it seems plausible and it is my only guess as to why it happens.
As for Corgar's specific post:
Short of having Fading Memories there is no easy way of clearing your aggro to loot the chests. You can zone out to clear your aggro, but if my theory is correct and you trigger a trap without it despawning (presumably after it lands), you will encounter the same problem.