How can one have hyperthyroid (graves disease) if it doesn't run in the family genes? What can possible cause it?

It is autoimmune. Graves disease is thought to occur due to a combination of factors. It is not well establlished, but the hypothesis are that some people have some genes which prime for autoimmune diseases, then something in the environment triggers the immune system to recognize the thyroid as foreigner. The immune system makes proteins that bind the TSH receptor, increasing the production of thyroid hormones.