Originally Jason brushes off the fact his mom died because he didn't want to talk about it. Bruce makes the deduction that she had an O.D. which turns out to be true.
Of course we got another writer and this was how she was said to die.
Was drug use suddenly too controversial? Was this part of the original idea to kill him off with AIDS?Could the Todds afford this health care? The change doesn't make sense and later went back to an O.D. but DITF had a lot of changes like implying in the art that Jason was more violent and was still pissed after saving Two-Faces' life.