It could be the cutbacks to the city’s transportation system, or a general decline in urban civility. Perhaps people are just in a collective bad mood.

What else could explain why New Yorkers — notoriously hardened to the slings and arrows of everyday life here — are spitting on bus drivers?

Of all the assaults that prompted a bus operator to take paid leave in 2009, a third of them, 51 in total, “involved a spat upon,” according to statistics the Metropolitan Transportation Authority released on Monday.

No weapon was involved in these episodes. “Strictly spitting,” said Charles Seaton, a New York City Transit spokesman.