Tenser was exposed. Everyone could see it. Overconfident because of his new wand, or maybe just exhausted from this gauntlet of a dungeon, Tenser stepped out from cover for a clearer shot with his wand. The command word for the wand was on the tip of his tongue -- but the dragon was one lunge away from biting him in half.
Fortunately, a lot of things then happened all at once.