Why did umar ra hit the servant?

Here it says: وقال الطبراني في الكبير 309 – حدثنا يوسف القاضي ثنا عمرو بن مرزوق ثنا شعبة عن سعد بن إبراهيم عن سعيد بن المسيب قال خرجت جارية لسعد يقال لها زيرا وعليها قميص جديد فكشفتها الريح فشد عليها عمر رضي الله عنه بالدرة وجاء سعد ليمنعه فتناوله بالدرة فذهب سعد يدعو على عمر فناوله عمر الدرة وقال : اقتص فعفا عن عمر رضي الله عنهما. The judge narrated: Amr ibn Marzuq told us: Shu'bah narrated from Sa'd ibn Ibrahim from Sa'id ibn al-Musayyib who said: A female servant of Sa'd, named Zira, went out wearing a new shirt. The wind blew at her so Umar (may Allah be pleased with him) struck her with his stick. Sa'd came to prevent him, so Umar struck him with the stick as well. Sa'd then intended to pray against Umar, but Umar handed him the stick and said, "Take retribution." However, Sa'd forgave Umar (may Allah be pleased with them both). What is the fault of the servent such that umar ra struck her?