Answer:
Helen of Troy was the indirect cause of the Trojan war when she ran away with Prince Paris of Troy away from her husband King Menelaus of Sparta who was the brother of the great king of Mycenae, Agamemnon.
Odysseus had once been her suitor so when he came with other Greek soldiers to defeat Troy, she swore not to betray him as he was in the Trojan Horse but still acted in the manner in the question.
She most probably did this because she wanted the Greeks to expose themselves by their own hand so that they may be found out and defeated, this way she would have not broken her promise to Odysseus and would not have to go back to Greece.