Yeah, that's been the issue past Summer too with Suzuki WagonR PKDM! Try to keep a half an inch window down when you leave out during summers, in daytime or try to park it under a shadow with no direct Sunlight!
It happens mainly due to heat and no trespassing of air in the cabin, the same reason why you sometimes have to close the doors twice because they were not correctly closed first time due to gentle push and air resistance in the cabin, when all the doors and windows are closed!
One more thing, 15 inch is a new thing though! People previously usually have had 3-4 inches crack in the windscreen during Summer! Try to be careful. All hail to the Pak Suzuki Motor Corp. 