Legal issues child custody?

Mother has had hard time, now that she has her life in order, the guardian she entrusted her child with is trying to have her rights as a parent terminated. She has went to extreme lengths, Doctors, private investigators and who knows what else. The child is now five and ( the mother of the boy ) has tried to get more visitation and the lady would always have plans, and it has been tied up in the court system for now close to three years. She has since had another child and does really well with her. I don't understand how can they terminate her rights as a mother? I'm not saying she's perfect but she is a very good mother.