It says in the article they had three dogs. The dead one was found locked in its crate inside a closed bedroom closet near Hackman’s dead wife, while the two that survived had the run of the house and presumably were able to drink enough out of the house’s toilets to survive until help came.
So, she probably put the dog in there for nap or something, then suddenly died before being able to let it out. And then the dog most likely died of thirst since the only other person in the house for the next week was a 95-year old completely debilitated with Alzheimer’s who subsequently died himself.
And if Hackman was that far gone, hunger/thirst and/or being off his meds probably contributed heavily to his death as well.