They are expanding T1 also T1 was operating at over capacity. So they needed to move some of the flight operations out till they can finish the expansion work. All 3 carriers operating out of T1 resisted the shift. It would have made much more sense if for instance only Indigo moved all it's operations to T2 - but they acted like major tools about moving.
Now this is going to create a lot of confusion for customers for sure. Despite all the efforts that the airport authorities and the airlines put in to make customers aware of their terminals etc. Indigo & Spicejet are now operating from all 3 terminals. A few people are going to miss their flights.