The pilots' decision to offload the cargo came as the Boeing 777, operating flight UA-49, was taxiing to the runway for take off soon after midnight, airport sources said.
They said the Mumbai-Newark flight, scheduled for 0025 hours, was "declared AOG (aircraft on ground, meaning cancellation of flight) after pushback and ramp return, after the commander decided that 13 tonnes of cargo had to be offloaded," they said.
When contacted, the airline blamed the 19-hour delay on inclement weather in the city and high winds, which led the pilots to decide on offloading the cargo.
In a statement a United Airlines spokesperson said: "The flight UA49 from Mumbai to New York/Newark on November 9, with 255 customers and a crew of 15 on board, was cancelled due to adverse weather conditions."