There is very strong documentation that the incidence of type 1 is increasing in children around the world, most quickly in children under 5 years old. Scientists agree that some environmental factors have to be causing the increasing incidence. Incidence means that more and more people are being diagnosed each year-- it's not due to living longer.
The genes the predispose to type 1 diabetes have become less common in the population than they used to be, according to a number of scientific studies. More people with only a moderate genetic risk are now developing the disease.
Different people can have different causes. I know someone diagnosed after a bike accident. I was diagnosed after pregnancy. My son was diagnosed at age 1 for no apparent reason. We'll never know. For most people, we may never know.
I wrote a website summarizing the scientific studies on the main environmental factors linked to type 1 diabetes: www.diabetesandenvironment.org. There is pretty good evidence that viruses, vitamin D deficiency, early diet and nutrition, a leaky gut, weight gain, stress, and some other factors can contribute to type 1 development. But none have really been able to account for the increasing incidence.
So I "stated my theory" in a peer-reviewed scientific journal (co-authored with a researcher who specializes in type 2 and chemicals) suggesting that the increasing incidence may in part be due to endocrine disrupting chemicals-- a factor that has not yet really been considered in type 1 diabetes research, but should be:
I'd guess that many of these factors could work together, along with genetic background, to trigger the disease in different ways in different people. Chemicals can interact with a number of factors, e.g., by interfering with vitamin D synthesis, increasing gut permeability, promoting weight gain... so I think that looking at the other factors without considering chemicals will be missing the whole picture.
I have a listserv on the topic if anyone is interested; see the "get involved" page of www.diabetesandenvironment.org.
Collaborative on Health and the Environment