By nature, Sītā is pure, chaste and ever dependent upon you. And by virtue of her ascetic dependence upon you she has gone to the region of Nagas.
सीता हि विमला साध्वी तव पूर्वपरायणा। नागलोकं सुखं प्रायात्त्वदाश्रयतपोबलात्॥