Mob casting spells midfight?

[cylynex2]

Trying to get an npc to cast poison storm (spellid 437) at a certain % hp. Everything works, he starts to cast the spell, but then interrupts and goes right back to meleeing. Any ideas? Here's the relevant piece of code:

        if ($hpevent == 40) {
		quest::emote("summons a storm.");
		quest::spawn2(39169,0,0,-446.3,577.5,-193.9,147.5);
		$npc->CastSpell(437,39168);
		quest::setnexthpevent(20);
	}