When asked about the mishap a moment later, he said sharply, "I don't fall down," then used an expletive to describe the agent who "knocked me over."
Notice the story doesn't say WHO asked Kerry about the fall. I'll bet it was this very reporter who asked Kerry about the Secret Service guy incident. But I'll bet he said something like, "Are you okay after your fall?" and Kerry didn't want it characterized as a failure or fall so he flips off the reporter and says his line. This probably ticked off the reporter enough to show Kerry as is instead of the usual NYT puff piece.