The crew of the aircraft made a forced landing because of a rowdy with a knife.

Indian Airlines plane was forced to make emergency landing due to poor passenger behavior, reports France Presse on Sunday, February 1.
Airbus A320 of Indigo Airlines with 150 passengers on board, flying an internal flight from Goa, made a forced landing at the airport of New Delhi, after one passenger started to riot.
As explained Reuters, the passenger said stewardess that he had a knife. The pilot reported the incident ground, and after landing, police officers detained a special rowdy. His name is not specified. How to explain the sources of AFP by the police, it was not a training concern.
Note that since the terrorist attacks in Mumbai, which occurred in November last year, the Indian air taken increased security measures.