caitanyera - of Lord Caitanya Mahāprabhu; avatāre - in the incarnation; ei - this; mukhya - principal; hetu - cause; bhaktera - of the devotee; icchāya - by the desire; avatare - He descends; dharma-setu - protector of religion.

Therefore the principal reason for Śrī Caitanya’s descent is this appeal by Advaita Ācārya. The Lord, the protector of religion, appears by the desire of His devotee.