The reason for Queen's nervous breakdown is not clearly stated in the miniseries. Critics usually claim that she had a fear of fire, which could actually make sense.
1- Once the war was over, the Yankee captain ordered for the Jackson's cotton to be burnt, putting at end to their way of life. It was the beginning of the end for Queen, since she had no place in the plantation anymore.
2-Then she witnessed a fire when she was first on her own and sought the help of a group of free slaves, who mocked her. Fate made her be hidden when the Ku Klux Klan members arrived and burned them all.
3-Later on, when Davis was lynched, he was also burnt and Queen found his body.
4-Finally, her gown caught fire when her sons were to live the family house.
However, I think that there is much more to it than a simple fear of fire:the most important thing about those fires is their causes and consequences.
1-She was asked away of the only home and the only way of life she had ever known, of the place where she belonged, of the family that was her own. In her own words: This was my home. My growin up, my life, my dreams and my nightmares. However, she wasn't loved, she didn't fit. Her mixed race condition got in the way, she was deprived of an identity, she didn't fit.
2- When she was free and on her own, she found out that the free slaves didn't recognize her as their own either, she wasn't wanted or allowed to stay, to join them, to seek for and give protection, to identify as a group. She wasn't comprehended. Her mixed race condition got in the way. She didn't fit. They fed her like an animal which ultimately saved her life.
3- Her first love was lynched because of the colour of his skin, and her child was endangered in the process. Mr. and Mrs. Benson didn't consider her an equal and used her to get to Davis. Her mixed race condition got in the way for her to fight to keep her child. She didn't fit.
4- Despite finally having found a place she could call a home, her sons wanted to leave the house. She was ok with Simon leaving so he could carry on studying and better himself, but she wasn't ready to let Abner go, she didn't want him to go through what she had to: starvation, fear, abuse and whatnot. In her words: He going for a purpose, for a reason. What are you gonna do? End up on the street like half of the nig-gers that go up north? Abner, you my first born, and I love you. But you ain't got no idea of what is like out there.
In sum, the departure of Abner and the accidental little fire, reminded her of all those other fires (and their causes and consequences) in her life. Causes that were never forgotten: I know I got a few demons in me, and I suppose they won't ever go away, but I ain't dangerous to nobody, except me..Consequences that made her grieve to the point of almost losing her mind. The constant struggle to find her place in the world, to fit, to be loved.