I am not sure if you are asking the cause of Mrs. Yeobright's death or if you are asking why Hardy chooses to have her die in the novel; so I'll answer both versions of the question.
1. Cause of death: Mrs Yeobright idolizes her son Clym and is, of course, unhappy when he chooses to marry such a controversial figure as Eustacia. After the marriage takes place, several events occur which serve only to strengthen Mrs. Yeobright's disapproval of her new daughter-in-law. One day, Clym's mother decides to travel to his cottage to make amends and to set things straight. However, Clym does not know that she has arrived at his door, and Eustacia does not open the door to her mother-in-law. When Mrs. Yeobright realizes that someone is inside and just refuses her entry, she leaves broken-hearted, believing that she has been rejected by her own son. She slowly tries to walk home but is overcome with the heat and her heavy heart. When she sits down to rest, she is bitten by an adder, and Clym later finds her, still alive, but she soon dies without knowing that her son did not reject her and, instead, tried to save her.
2. Hardy's choice to kill off Mrs. Yeobright's character: Mrs. Yeobright's death is simply another view of Hardy's fatalistic philosophy that Providence flirts with humans but does nothing to intervene on their behalf. The author would argue to his critics that it was Mrs. Yeobright's fate to arrive when her son was asleep and for her to be killed by the bite of an adder. Life on the heath is difficult and unforgiving, and Mrs. Yeobright's death not only exemplifies that truth but also serves to propel the tension between Clym and Eustacia, leading to the novel's tragic ending.