The Lord tried to conceal His identity as the Supreme Personality of Godhead by representing Himself as a devotee, but His pure devotees could recognize Him by His special features. The Vedas and Purāṇas foretell the appearance of Lord Caitanya, but still He is sometimes called, significantly, the concealed descent of the Supreme Personality of Godhead.
Advaita Ācārya was a contemporary of Lord Caitanya's father. He felt sorry for the condition of the world because even after Lord Kṛṣṇa's appearance, no one had interest in devotional service to Kṛṣṇa. This forgetfulness was so overwhelming that Advaita Prabhu was convinced that no one but Lord Kṛṣṇa Himself could enlighten people about devotional service to the Supreme Lord. Therefore Advaita requested Lord Kṛṣṇa to appear as Lord Caitanya. Offering tulasī leaves and Ganges water, He cried for the Lord's appearance. The Lord, being satisfied by His pure devotees, descends to satisfy them. As such, being pleased by Advaita Ācārya, Lord Caitanya appeared.