Lots of evidence supporting the hypothesis that Diabetes 1 is an auto-immune disease.
The pathogenesis is essentially that the cells responsible for producing insulin are attacked and destroyed by the bodies own immune system. The evidence supporting the autoimmune hypothesis is plentiful and multifactorial, ranging from positive family history, through to associations with other autoimmune diseases,an association with HLA class II markers,, observed lymphocytic infiltrationin the islets of langerhans, and the observation that treatment with immunosuppressive drugs retards the development of Diabetes 1. It is also characterised by the presence of autoantibodies in the serum, specific for insulin and lnagerhans islet cells. Further evidence comes from a whole host of animal studies.
So, there is no serious evidence suggesting that Diabetes 1 is anything other than an autoimmune disease.
As to triggers - well, thats always a little uncertain, as sqad has already alluded to. Individuals who go on to develop diabetes 1 almost certainly have an inherited genetic predisposition. The actual trigger is less clear, but there is some circumstantial and inferential evidence to support the idea that there may be more than one type of trigger, ranging from viral infection, through to exposure to chemical toxins. The idea that early exposure to cows milk is a cause ( ie early cessation of breast feeding) is very controversial, with the evidence supporting the hypothesis marginal and subjective.