Malbim: The nation accepted three matters with a Shevu'ah. They brought Ma'aseros, guarded Shabbos, and separated from their Nochri wives. They all transgressed these when Nechemyah left Yerushalayim; now he returned them to observe them. The first time, they accepted via Ezra, or via themselves. It is attributed to Nechemyah, for after they were breached, he fixed them. He requested that this be a merit for him, as if he did it all from the beginning.