The Champs do not use traditional aggro, they instead follow targeting scripts.
Obviously I don't know the script, but there are some features I have figured out:
1.) Taunt will make you their target for the length of the fixation (with diminishing duration, so 3 sec > 1.5 sec > 0.8 sec > 0.5 sec > immune roughly)
2.) They will pick a target and attack it, but certain elements will change their mind. If they are CC'd, if their target is healed, if their target drops below a certain % health, they will behave differently, either picking a new target or staying with their current target.
To be more specific. Stun/CC/Incap's that make the target inactive the NPC's drop targets, so they will start fresh when they come out. That sometimes means they'll pick the same target but it's not for sure.
They do track healers. We had an early wipe in figuring them out where we had a few people being the first in to get their attention, these people got hit, got healed, then the champs turned around and wiped out our healers. They don't show undue attention to healers, but there is an element in the script, I'm fairly certain (if not it's a remarkable coincidence).
The way they change healing priorities and heal low health targets harder, they also like it when you're lower on health and will stick to you longer. If you get healed and stay at a higher health they'll eventually decide to find a new target. It's chaotic so it's hard to say what their threshold is, but I'd guess it's reasonable to assume around 35% health (extended execute range).
Threat meters will be misleading. Old school style meters use client side formulas to tally up your threat and as such the number is meaningless. Most in-game meters now take cues from the built-in threat system and so probably say weird things when dealing with these mobs who don't pay attention to threat the same. Most out-of-game meters will work the same as the old threat meters and measure based on threat formulas and ability damage, and again, with the chaotic nature of the fight, you wouldn't be able to glean meaningful info without going through the combat log line by line.