Why was Darui sucked into the Benihisago?
Taken from the Naruto wiki, 'he inadvertently uttered the word when he admonished the brothers and stated, "Ore wa Bosu no migiude da. Ruijihin janē! (「オレはボスの右腕だ。類似品じゃねェ!」, I'm the Boss' right arm. I'm no imitation!)"'. "Daru" sounds like his most said word, "darui."
This is why he was sealed.