Continuity mistake: When the Hulk throws the tire during his second transformation, the first shot shows a horizontal trajectory but the second shot shows a 45 degree angle trajectory, which makes the tire land much further than expected.
Continuity mistake: When David is leaving at the end, he is wearing a blue shirt under his coat. Moments later, he removes his coat but he is wearing a different color shirt with a square motif.
Continuity mistake: When the Hulk throws Mr. Malone in the river during his second transformation, the first shot is too long and we see that he would fall a few feet from the closest river shore. But in the second shot he is flying like crazy and almost lands on the remote river shore.
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