Environmental. No one has proven that there are environmental causes per se. Theories exist regarding soy products which may contain phyto- estrogens. Another common theory is that hormones are placed in meat or milk. All of these are conjecture and really have no basis in fact. One proven association is weight gain and obesity in girls which predisposes them to developing early.
Precocious puberty. Precocious puberty is pubertal development before age 8 in girls and age 9 in boys. The causes are either gdpp) early developmental signals from the brain, or gipp) abnormal steroid production such as from tumors or ingesting hormonal medication. Environmental causes would fall under gipp, such as ingesting drugs or material containing sex-steroid hormones.