i just realized that it's likely because the npc's target is at the top of the hate list (the first entity in the array), and the loop you have is trying to get the npc to cast the spell on a new target while he's still casting on the first one.
you could attempt to address this by putting a delay (sleep($seconds)) equal to the cast time of the spell in the loop (you'd want to place it after the casting has begun). you could also check $npc->IsCasting() periodically to see if you needed to continue on to the next target. that way you wouldn't have a lengthy delay between casts if the cast was ever interrupted by the players. i'm not sure what sort of load that would cause on the server, however.
DUH.
or you could set the cast time to 0 in CastSpell()... i forgot that one.
CastSpell(spell_id, target_id, slot= 10, casttime= -1, mana_cost= -1)
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