Detailed mechanism of the cause of diabetes mellitus type 1?
Diabetes mellitus is divided into four  type 1, type 2, gestational diabetes, and specific type of diabetes. Type 1 diabetes is commonly called as Juvenile diabetes or Brittle diabetes.
It is also called as "Insulin-dependent diabetes mellitus (IDDM)". For our bad-luck the causes of Type 1 diabetes remains obscure, and scientists believes its mostly related to genetics and other environmental factors like mutations or virus attacks. I found some articles that may help.


  But it
  is believed that genetic and environmental factors (possibly viruses)
  may be involved. The aetiology ('cause') of type 1 diabetes is
  unknown, but a good deal is known about its pathogenesis (the way in
  which it develops). As with other complex diseases, the outcome is
  determined by the interplay of multiple genes and (most likely)
  multiple environmental determinants, together with an element of
  happenstance. The risk of developing diabetes is strongly influenced
  by genes affecting immune function, particularly the HLA system,
  but other factors are involved. The best evidence for this comes from
  the study of identical twins: if one twin develops diabetes in
  childhood, the other (who has identical genes) has no more than a one
  in three chance of developing the disease. Prospective studies in
  human populations reveal that circulating auto antibodies directed
  against the islets typically appear in the first 5 years of life, and
  may be present for many years (sometimes 20 years or more) before the
  disease develops. A similar latent period is seen with coeliac
  disease, and implies that some sort of regulatory balance has been
  achieved within the immune system, and subsequently lost. This opens
  the door to the possibility of 're-education' of the immune system by
  various forms of immune intervention.
  Reference
  
  The pathogenesis of Diabetes mellitus type 1 shows a strong tendency towards genetic related factors,
  
  Reference