It is possible that all autoimmune diseases are caused by viruses.
I can see two ways this could happen. These are my ideas only. As far as I know, there is no proof or evidence that they are true.

1. The virus invades the DNA of the person's immune system cells and makes them go haywire.

2. The virus attacks other tissues (in our case the neuromuscular junction) and in the immune system's efforts to rescue us, it accidentally starts attacking our own cells.

Kinda like a dog that is killing the fox in the hen house and then develops a taste for chicken.

Once a dog starts to eat chickens, it is hard to break him of the habit.