This crying of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam was due to the listening of the words of Allah, and this befits the dignity of the Qur-aan. Imaam Nawawi writes that to cry whilst reciting the Qur-aan is the quality of the 'aarifeen and a sign of the saaliheen. Allah Ta'aala has praised these people in the Qur-aan at many places. Allah Ta'aala says: "When the
revelations of the Beneficent were recited unto them, they fell down (into sajdah), adoring and
weeping".-Surah Maryam, 58.

"Lo! those who were given knowledge before it, when it is read unto them, fall down prostrate on
their faces, adoring".-Surah Israa (Bani Israa-eel), 107.

By this it.is clear that who can confirm more to these aayaat than Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam. It is not imaginable that the crying of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam was due to the greatness and Jalal of the Qur-aan. In this case, it is possible that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam must have been crying from before and Sayyidina 'Abdullah bin Mas'ud Radiyallahu 'Anhu must have observed it only after Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam had been crying for some time. It may also be possible that he began crying specially after the reciting of the aayah. The translation of this aayah is:
"But how (will it be with them) when We bring of every people a witness, and We bring thee (0
Muhammad) a witness against these?'-Suratun Nisaa, 41.
 

Sayyidina Rasulullah Saliallahu 'Alayhi Wasallam will be called to give evidence on these people. This is the scene of qiyaamah. On that day every nabi will be called upon to give evidence as a witness. The events of which are described in detail in the ahaadith of the day of mah-shar. In this manner it is possible that this crying of Sayyidina Rasuluilah Sallallahu 'Alayhi Wasallam will be due to the difficult times on the day of qiyaamah and mah-shar. Every person will be engrossed and worried of one's self only. It is possible that in this aayah Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam is being commanded to appear as a witness. And he fears the intense burden of being called up as a witness. It is stated in a hadith that at the time when this aayah was revealed, Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam said: 'O Allah, how can I be a witness of those who were not in my presence?' Some of the 'ulama have written that at the mentioning of this aayah, the scene of qiyaamah came before him, and the fikr of the ummah was always with Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam. He may have cried because of the sins of the ummah. For this, every event is such, that it can be the cause of crying. All this and besides this, there could be other reasons too for his crying.