Flight of Sukhoi-30 and Mirage 2000 fighters, how the accident happen?

Sukhoi-30 and Mirage-2000 aircraft flying from Gwalior airbase collided with each other and crashed. Mirage-2000 is the deadliest aircraft of the Indian Air Force. It was inducted into the Indian Air Force in 1985. The Sukhoi-30 MKI has been considered the backbone of the Indian Air Force.