My theory on what happened with them: she and her ex either fell out of love or didn't really love each other. The husband is a workaholic (according to her) and probably left taking care of the household to her. On top of that, her MIL is probably overbearing and criticized her often. She lives close by too so she probably visits often, not giving her privacy. Likely the stress from all of it caused her to be less patient with her child, and when she almost hit her daughter out of anger, she realized this isn't working out anymore and filed for divorce.
The daughter is still seeking attention from her mom. She knows her mom has anger issues but she probably never hit her as she doesn't fear her.