The answer is that we don't precisely know, so we're taking a very wide-angle lens.
Environmental triggers are being looked at. There was a study, for example, published last year from China, where type 1 diabetes is occurring. Bisphenol A, the stuff in hard plastics, might be a risk factor for diabetes.
There is some evidence—actually excellent Canadian work—suggesting that it's not chemicals in the environment, but it's the composition of the microbes that colonize your intestinal tract that are a risk factor. Another one might be the kind of dietary exposures you have early in life.
So all of those things are being looked at, at the same time, and we don't yet have the answer. But it's a wide angle, because if you go too narrow you might miss something that is the real culprit.