Nil kir and Municiva, the sense Kuver, were great devotes of Lord Śiva, bur because of material opulence they became so extravagant and senseless that one day they were
enjoying with naked girls in a lake and shamelessly walking here and there. Suddenly Nārada Muni passed by, but they were so maddened by their wealth and false prestige that even though they saw Nārada Muni present, they remained naked and were unashamed.
Due to opulence and false prestige, they lost their sense of common decency. It is the nature of the material qualities that when one becomes very opulent in terms of wealth and prestige, one loses one's sense of etiquette and doesn't care about anyone, even a sage like Nārada Muni. For such bewildered persons, who especially deride devotees, the proper punishment is to be again stricken with poverty.
A poor man can be easily convinced that the prestige of an opulent position in this material world is temporary, but a rich man cannot. Therefore Nārada Muni set an example by cursing Nalakūvara and Maṇigrīva to become trees. This was a fit punishment. But because Kṛṣṇa is always merciful, even though they were punished they were fortunate enough to see the Supreme Personality of Godhead face to face. Therefore, the punishment given by Vaiṣṇavas is not at all punishment; rather, it is another kind of mercy. By the curse of the devarsi, Nalakūvara and Maṇigrīva became twin arjuna trees and remained in the courtyard of mother Yaśodā and Nanda Mahārāja, waiting for the opportunity to see Krena directly. Lord Kṛṣṇa, by the desire of His devotee, uprooted these yamala-arjuna trees, and when Nalakūvara and Maṇigrīva were thus delivered by Kṛṣṇa after one hundred years of the devas, their pure consciousness was revived and they offered Kṛṣṇa prayers suitable to be offered by demigods. Having thus gotten the opportunity to see Kṛṣṇa face to face, they understood how merciful Nārada Muni was, and therefore they expressed their indebtedness to him and thanked him. Then, after circumambulating the Supreme Personality of Godhead, Krena, they departed for their respective abodes.