New York City was called “Strike City, U.S.A.” in October 1968 because of so many municipal strikes. The nickname “Strike City was influenced by Mayor John Lindsay‘s “Fun City” nickname of 1966.
 
The “Strike City” nickname has been rarely used since the 1960s.
 
 
10 October 1968, Richmond (VA) Times-Dispatch, pg. A-12, col. 1:
‘Strike City, U.S.A.’
New York’s MAYOR JOHN V. LINDSAY used to be fond of referring to his metropolis as “Fun City.” But you don’t hear that very often nowadays. “Strike CIty, U.S.A.” has become a far more appropriate nickname.
 
Last month, when New York’s teachers walked off the job, no one was very surprised—for teachers’ strike have become an annual affair in Gotham. Actually, they’ve suddenly become much more frequent. The city’s 1.1 million children have lost more than two weeks of school so far this year as a result of two strikes, not one, and a third walkout by the teachers was threatened, but apparently has been averted.
 
22 December 1968, New York (NY) Times, pg. SM7:
Why New York Is ‘Strike City’
Lindsay’s golden-boy image has been a strike victim, too
This New Year’s Eve a whole new series of city labor contracts will expire
By A. H. RASKIN
IN one of a thousand dark hours of labor crisis at Gracie Mansion, width the city’s teachers on strike and the police, the firemen and the garbage collectors all making menacing noises, a mediator came upon Mayor Lindsay with his head sunk in his hands. “
 
15 March 1969, New York (NY) Times, “The Antilabor Mood Reflected” by Peter Millones, pg. 44:
... and for assorted strikes that have led some to call New York “strike city.”
   
New York (NY) Times
When Revolt Was Order of the Day
By LAWRENCE VAN GELDER
Published: December 29, 1996
In 1968, there were 986 homicides in New York City. No one knew it then, but 28 years would pass before the figure was below 1,000 again. Through the mists of time, 1968 might appear a halcyon year.
 
Far from it.
(...)
In February, 10,000 New York City sanitation workers struck for nine days. In the autumn, police and firefighters, also unhappy with their contracts, began slowdowns. Some people were calling New York ‘‘Strike City.’‘
 
New York (NY) Times
Herbert L. Haber, 89, Dies; Labor Negotiator in a Strike-Torn Era
By PAUL VITELLO JAN. 26, 2014
Herbert L. Haber, the chief labor negotiator for the City of New York from 1966 to 1973, when strikes by transit workers, firefighters, the police, teachers and garbage collectors tested the balance of power between the city and its municipal unions, died on Jan. 20 in Auburndale, Mass. He was 89.
(...)
There was a near-perfect storm of coinciding currents, he said — the rising power of municipal unions, the city’s declining prosperity, the ’60s zeitgeist, and the visceral distrust between union leaders and a patrician Republican mayor — contributing to the events that earned New York the nickname “Strike City,” during the Lindsay years.