Jenny suffered from child abuse. Many sources quote or explain that, such as this fandom :
She was raised by her father, a farmer, who physically and sexually abused Jenny and her sisters. Forrest, being simple minded, believed that he was simply a loving father as he was always kissing and touching Jenny and her sisters.
Since the film is told through Forrest's eyes, we only get limited glimpses of this, but, even untold, it's clearly the case. There's a scene where, as a child, she runs away from the house and her father and they (she and Forrest) hide in a field.
Forrest: "He [Jenny's father] was a very loving man, always kissing and touching her and her sisters.”
When, as an adult, she goes back to this place, she realizes all the pain she suffered because of her father in this very same house. She's overwelmed by emotions and she wishes she could just erase the past by destroying the house.
I like the way this site tells the story:
One day, while on a walk, the pair happen to stumble upon Jenny’s abandoned childhood home. After walking up to the house, Jenny stops and thinks for a bit, reflecting on what had happened to her as a child. Finally, she sees an opportunity to let her anger out and goes for it, feverishly throwing her shoes and rocks at the house — hitting boards and breaking windows. While throwing objects, she says things like, “How could you” and “How dare you” in the process.
It’s suggested that in that moment, Forrest realizes the truth about what Jenny’s father did to her, and that Jenny understands that her upbringing was the catalyst for her painful and rocky years. In her fit, she falls to the ground, sobbing. Forrest quietly walks over and sits nearby, and (in his narration) says, “Sometimes I guess there just aren’t enough rocks.”