 इदं तावद्यथाकाममीश्वरस्य कृते कृतम्।रामस्य तु परित्यागे न हेतु मुपलक्षये।। 
 Or he might have thought he is the overlord and can do as he likes. Or, he might have done this for fear of God Otherwise, I do not see any reason for the abandonment of Rama.  