I think for 80 out of 100 (eighty twenty rule), the cause of Type 2 diabetes is insulin resistance. For me the imbalance of carbs / sugars and vegetable oils over a number of years, overwhelms the pancreas with too much work to do, which in turn keeps insulin too high, causing resistance and fatty liver. There is a stark contrast to pre-1980 eating guidelines and for example in the UK 1970's consumption of what used to be called "meat and two veg". Back then meals were 2 to 3 a day, lower GI load, more local and home made predominantly.
There are several graphs that show correlation almost down to the month in the rise of several metabolic conditions when low fat, grains and sugary higher carbs achieved the industry backed upper hand over higher fat regimes. Whilst correlation does not show causation (yawn) again there are so many variables that are the same for differing societies which adopted the SAD diet.
For me this is clear cut Occam's razor territory. We know what changed from 1977, we know that sugars increased the most, we know sugar creates fat (obesity coincides 80% of the time), we know sugar rots teeth, we know carbs raise blood sugar. I feel there are so many singing canaries in favour of sugar being the main culprit (by sugar am interchanging carbohydrates which are higher GI).
I am firmly with Gary Taubes et al - sugar / carbs and vegetable oil and the so called foods made out of this combination are my assessment for the majority of cases, others have a different opinion, but due to the almost uniform nature of around 10% diabetes rates across societies, meats, old time oils / fats declined over the same time period metabolic explosions have taken place.