Continuity mistake: During the flashbacks of the abusive father's childhood his own father's shirt sleeves go back and forth from rolled up to just unbuttoned.
Revealing mistake: When The Hulk tosses the abusive father across the gym his blond hair looks fluffier and darker, indicating it's a wig on a stuntman.
Answer: An illness, although it's never stated what kind of illness. David blames his dad for her death because his father never took her to the hospital. So in all likelihood, her illness was never diagnosed anyways and David might not even have known what it was.
Bishop73