This came up for Void Reaver but it applies to all encounters where mobs have abilities to reduce their targets threat. The question is basically where does the mob go once the threat on the one highest on the list is reduced. Does it prefer melee first and then the ranged 130% rule applies? Or it just goes to the one highest on the threat list right after the threat reduction. Just to make it more clear assume:

Tank1: 20000 threat
Ranged1: 12900 threat
Tank2: 10000 threat

Void Reaver knockbacks Melee 1:

Ranged1: 12900 threat
Tank2: 10000 threat
Tank1: 4000 threat

Where does Void Reaver go? Ranged1 or Tank2?

If the answer to the above is Ranged1 than what happens if Ranged1 has 10900 threat? Any changes?

There are no special agro transfering mechanics associated with a knockback. The knockback knocks you away from the NPC and reduces your threat. Now you are at range with a new threat value. If someone in melee range is 10% above you, agro transfers to them. If a ranged person is 30% above that person, agro moves to the ranged person. On mobs like Void Reaver I've maintained agro through 2-3 consecutive knockbacks because nobody was close to me on threat so he just chases me after the knockback and I keep tanking.

It doesn't really matter in this scenario. Tank2 is > 110% above Tank1 and Ranged1 is < 130% than Tank2. Just assume that Tank1 after the knockback is lower than everyone.

This.