If I had to make guesses, my take is a viral+immune system interaction. You get the virus which causes your own immune system to respond. In that response we get a 'partial match' to something inside of our own system. This leads to a low level inflammatory attack against your own system.

In type 1 diabetics there is some evidence of a post viral auto-immune cause in some cases.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2570378/