Paper lanterns rushing to the scene of a fire; riverbank fog
chōchin ga kaji ni tobu nari kashi no kiri
(Ozaki Hōsai 尾崎放哉)
The word nari functions as a copula (similar to “is”), with everything preceding it being subordinated to an implicit noun such as “a situation”, “a matter”, etc. This construction is commonly used for giving an interpretation or explanation of observed facts, and a rough English equivalent would be: “what is happening here is that lanterns are rushing to the fire”. The explanatory tone is necessary in this case because the fog makes it impossible to directly see what is going on.