How does stress cause hair loss ?

is there a chemical imbalance somewhere ?How does stress cause hair loss ?
Yes, the more stress the more hair falls out. 3 years ago my brother killed himself, my husband moved out, my mother contested my brothers will, and my dad had surgery and came to stay with me. I lost about half of the hair on my head. Good thing I had enough for 2 people or I'd be bald now.