Why is my horse's mane falling out? ?

My horse was at a trainer for a week and a half before I pulled him. I didnt not like his methods, and my beautiful Palomino looked freaked out and chewed up from all the other horses. He was even hobbled with a rope! To make a long story short, i took him home immediately losing all the money I had put towards his training. When i got home i brushed him and noticed his mane is falling out by the hand fulls? Could this be because of the stress? I really need advice on what i should do?