I conceive that while taking Sītā, the Rākşasa darted upward, he being influenced by fear of Rāma's shafts; (and accordingly losing control over his movements), she fell off by the way.
क्षिप्रमुत्पततो मन्ये सीतामादाय रक्षसः। बिभ्यतो रामबाणानामन्तरा पतिता भवेत्॥