In the explanation transmitted from Jalal al-Din al-Suyuti, the etiquette of Al-Hujurat is also applied to greeting the Prophet ﷺ near his blessed resting place. One should offer greetings with humility and a lowered voice.
Raising one’s voice unnecessarily or speaking without respect conflicts with the Prophet’s honored station. Reverence is connected here with the discussion of his special life in barzakh.