What causes grave's disease to happen?

Autoimmune. Hyperthyroidism is most common feature of Graves' disease, affecting nearly all patients, and caused by autoantibodies to thyroid stimulating hormone (TSH, thyrotropin) receptor (TSHR-Ab) that activate receptor, stimulating thyroid hormone synthesis and secretion as well as thyroid gland enlargement (causing a diffuse goiter).
Autoimmune. You have hla antigen that predisposing you to getting autoimmune disease (likely inherrited from parent). Usually there is a major stressor in your life that triggers the activation of the smoldering beast. Once the immune response is triggered, you develope stimulating antibodies against your thyroid gland telling it to turn on and make inappropriate amounts of hormone.