Answer
Abdrabbuh Mansur al-Hadi resigned as president and fled Yemen due to the Houthi capture of Sanaa, the country's capital. The Houthis, a Zaidi Shia Muslim group, had been waging a civil war against the Yemeni government since 2004. In September 2014, the Houthis launched an offensive on Sanaa, quickly capturing the city and forcing al-Hadi to flee to Saudi Arabia.