To elaborate a bit on the above, here's what happens.

  • Main selects target and can now interact with the new target.
  • Main wow sends message to Blizzard server saying "Hey, I selected a new target."
  • Server acknowledges new target selection.
  • Server notifies Main and any clients in range that you have a new target
  • Other clients in range receive notification that you have a new target and can now use it.
The lag between the second and last bullets above is where your problem is.

NOTE: If you forget to remove the "/assist focus" from your main, your main will suffer the same exact problem as your alts.