I wonder if there have always been people in clown suits brandishing knives and peeping in windows (or at least a steady rate of menacing clowns, say 1:1000 clowns?). The Northampton clown guy only brought attention to clowns and so now your run of the mill clown menacing is being portrayed as a trend.
It doesn't seem likely to grow much. It is not like the police would have a hard time finding a clown post-incident. Clowns can't blend into a crowd very well. Unless they are well connected in the Buffoonish Underground, and then we have a whole different level of manically smiling shit to deal with. Have you seen how many they can fit into a single vehicle?
Watch out for things that go honk, or possibly squeak in the night.