Bob, MG is an autoimmune disease. At the moment the medical profession doesn't seem to have much idea what can cause the immune system to go haywire. And in fairness there seem to be a multitude of ways that it can go wrong. And therefore an even greater number of probable causes.

If anyone in your family has had any autoimmune disease, then it seems that you are more likely to have one. That is not hereditary, but suggests that a susceptibility to such things may be hereditary.

In practice no one knows.