Isn't it just because the targeted mob dies, your hunter gets attacked again and therefor auto selected ? When a player or mob begins combat towards you with any action, you will auto select that person or mob. You can't turn it off iic, but no problem right ? Just get a macro to target your first hunter...?

/target (insert main name)

/assist

something like that, easy

Btw I don't think an addon does the targeting for you, because then you would of been banned already