Why did Allah Almighty say: (And you did not expect that the Scripture would be cast upon you) using 'cast upon' instead of 'sent down,' and how was it cast?

Allah Almighty sent down His Book in a clear Arabic tongue, and it was a miracle with which He challenged the Arabs in their eloquence. And among the wonders of its style is its conveying of a single meaning through various expressions, as in His Almighty saying: "And you were not hoping that the Book would be cast upon you." Here, "cast upon" means to inform and to send down, meaning that the Prophet, peace and blessings be upon him, did not aspire for revelation to descend upon him before his prophethood, and it was a mercy from his Lord. Likewise, he did not guarantee the continuation of this revelation after his prophethood, as in His saying: "And if We willed, We could surely take away that which We have revealed to you," and this resembles His saying: "You did not know what the Book was, nor what faith was," meaning that the revelation descended upon him as a mercy from Allah to him and to His servants.