Vasuki, being a great snake, would be natural food for Garuda, the chief of birds. Lord Visnu therefore asked Garuda to leave so that Vasuki could be brought to churn the ocean with Mandara Mountain, which was to be used as the churning rod
SB Canto 8
Thereafter, Garuḍa, the chief of birds, unloaded Mandara Mountain from his shoulder and brought it near the water. Then he was asked by the Lord to leave that place, and he left.
Garuḍa was asked by the Lord to leave that place because the snake Vāsuki, who was to be used as the rope for churning, could not go there in the presence of Garuḍa. Garuḍa, the carrier of Lord Viṣṇu, is not a vegetarian. He eats big snakes. Vāsuki, being a great snake, would be natural food for Garuḍa, the chief of birds. Lord Viṣṇu therefore asked Garuḍa to leave so that Vāsuki could be brought to churn the ocean with Mandara Mountain, which was to be used as the churning rod. These are the wonderful arrangements of the Supreme Personality of Godhead. Nothing takes place by accident. Carrying Mandara Mountain on the back of a bird and putting it in its right position might be difficult for anyone, whether demigod or demon, but for the Supreme Personality of Godhead everything is possible, as shown by this pastime. The Lord had no difficulty lifting the mountain with one hand, and Garuḍa, His carrier, carried all the demons and demigods together by the grace of the Supreme Lord. The Lord is known as Yogeśvara, the master of all mystic power, because of His omnipotence. If He likes, He can make anything lighter than cotton or heavier than the universe. Those who do not believe in the activities of the Lord cannot explain how things happen. Using words like "accident," they take shelter of ideas that are unbelievable. Nothing is accidental. Everything is done by the Supreme Personality of Godhead, as the Lord Himself confirms in Bhagavad-gītā (9.10). Mayādhyakṣeṇa prakṛtiḥ sūyate sacarācaram. Whatever actions and reactions occur within the cosmic manifestation all take place under the superintendence of the Supreme Personality of Godhead. However, because the demons do not understand the potency of the Lord, when wonderful things are done, the demons think that they are accidental.