Why did the Throne of the Most Merciful shake for Sa'd ibn Mu'adh? Thank you.

Al-Bukhari and Muslim narrated that the Prophet (peace be upon him) said: "The Throne shook at the death of Sa'd ibn Mu'adh." Ibn Hajar interpreted the shaking of the Throne as its rejoicing and delight at the arrival of his soul, and he supported this with Al-Hakim's narration from Ibn Umar with the wording: "The Throne shook with joy for him." It was also said that what is meant by the shaking of the Throne is the shaking of its bearers, or that it is a sign installed by Allah for His angels to make them aware of the virtue of His saints.