Well, how i understand it currently, the T-cells need to recognise the antigene or antibody of a virus or cell first to attack them, they can find these antigenes and antibodies themselfs, or they can be presented to them by another cell.

So if the T-cell has a mutation and decides one of your cells antigene means trouble, they will attack. But that is just 1 cell, so it wont cause an autoimmune disease. That can also happen from overpresentation (for example if you have a little to many bacteria in your microbiome), wich is when it CAN cause an autoimmune disease, because its a lot of cells that go against you.