"It is known that the cause of insulin resistance is obesity but are there other reasons ?"
I am a T2 and although I was a "chubby child" I slimmed down in my early teens and have never been more than a few pounds overweight since then as I always refused to buy bigger jeans so as soon as they were too tight I lost the few extra pounds.
I am sure mine was life style. Stressful job, not the best of diets and I am sure a big factor was chronic lack of sleep so you don't even have to be overweight to be insulin resistant. I had a liver and pancreas scan (just the one done with the hand held scanner) and was told that I didn't have a fatty liver or pancreas.