Why did the Prophet's eyes ﷺ fill with tears when the verse: "So how [will it be] when We bring from every nation a witness and We bring you, [O Muhammad] against these [people] as a witness?" was recited to him?

The weeping of the Prophet, peace and blessings be upon him, upon hearing the verse, was due to the immense dread and gravity of what it contained regarding the horrors of the Resurrection and the intensity of the matter, as the prophets will be brought as witnesses over their nations, and he, peace and blessings be upon him, will be brought as a witness on the Day of Judgment.