سَقَطَ الْجِسْرُ. جَرَفَهُ السَّيْلُ.
The bridge collapsed. A flood swept it away.
If the collapse is unexpected news, a listener may naturally ask, 'Why did it collapse?' or 'How did that happen?' The second sentence answers this implied question and can be treated as explanatory resumption. Reconstructing the question explains coherence; it does not claim an actual spoken exchange.
If the discourse is already cataloguing flood damage, the latter clause might instead specify the earlier information. The strongest analysis is the one that derives the question from context and verifies that the new clause answers it.