When Toth jumps to Zorn it applies the Fearful debuff potential on Zorn. (( You can watch his buff/debuff bar, a countdown timer for Fearful starts the second Toth lands,)) If you damage Zorn and are below the 30 meter range during that time, you'll get the debuff and have to swap targets.
If you're out of the 30 m range when the jump occurs and then damage him, you won't get the debuff.
I've avoided Fearful by dotting Toth after the jump (2 GCDs), then return to Zorn. The only time I actually move is when you have to. (red circles // yellow circle)
Also: The Bioware development team will not comment on Boss Mechanics. I sent them a message once asking about using extrication on the person that has the bomb during Kephess. I feel it's an exploit because the player can be extricated well before he ever plants the bomb up against the walker. (Once the cable goes up, you can use it and the bomb still goes off.) If the player never makes it to the walker, why should it blow up? It's easily not working as intended.
I wasn't answered due to "Not being allowed to discuss boss mechanics".
Even something as small as that, couldn't be clarified on. It's ridiculous. Thankfully, I run with healers who aren't terrible and can heal the fight properly.