The legality of Cortes' position in Mexico was questionable at best, if not flat out treasonous. His personal and professional quarrels with Diego Valazquez had reached a boiling point just before his expedition left Cuba- Governor Valaquez had in fact fully revoked Cortes' exploratory charter by the time he departed. Therefore, Cortes and his men were, in the Governor's view, essentially outlaws.

Knowing that Valazquez would bring the issue to the attention of the crown, Cortes needed to play damage control in a sense. After arriving in Mexico, he sent a series of letters to King Charles V outlining (and it seems likely exaggerating) his findings in Mexico. Fortunately for Cortes, the monarchy was heavily inclined towards greater expansion of the colonial empire, which made his account all the more alluring. The letters are interesting to read as Cortes tries everything he can think of to entice the imagination of the court. In the second letter, he even compares Mexico to the recently reconquered areas of Spain, saying

"it is larger than Granada, and much stronger, and contains as many fine houses and a much larger population than that city did at the time of its capture; and it is much better supplied with the products of the earth, such as corn, and with fowls and game, fish from the rivers, various kinds of vegetables, and other excellent articles of food".

He's drawing a parallel that strikes a chord in the Spanish mind between colonial expansion and the history of the reconquista that Charles could not have missed. Furthermore in the letters, Cortes needed to assure the King that the crown would receive the customary 1/5th of the booty, and also generally make assurances that he was and always would be loyal. The letters served to legitimize and publicize his efforts in Mexico. It was a form of personal propaganda.

I'm not sure what Valasquez wrote to the King, but Cortes' letters clearly worked; after the conquest, he was made Governor and Captain General of New Spain (though even that was not enough for Cortes, who continued to badger the crown for more straight up to his death) and was not prosecuted for his misconduct.