'Reasons' seem clear, exact mode of death not.
Malerao died within a year after his succession, being affected by insanity.
— N. N. Nagrale: "Ahilyabai and her Benevolent Administration", Proceedings of the Indian History Congress , 1979, Vol. 40 (1979), pp. 700–706 (jstor)
As is repeated here:
On 5 April 1767, Male Rao sank into insanity and died within a year of succession.
She began to assume more and more of the responsibilities of rulership. Malhar Rao's death soon followed, and Ahalyabai's son Male Rao was named head of the government, but he was mentally unstable and died shortly thereafter.
— Kathryn Hansen: "The Virangana in North Indian History: Myth and Popular Culture", Economic and Political Weekly, Vol. 23, No. 18 (Apr. 30, 1988), pp. WS25-WS33 (jstor)
But this all too convenient, is it:
At first, Ahilyabai’s son Male Rao Holkar was appointed as the ruler, with Ahilyabai as the regent. But his reign lasted only a few months before he died. Male Rao was apparently insane, cruel and incompetent, depending on which accounts you read. In one story, he put scorpions into the shoes of some courtiers; they were bitten and died. For this, Ahilyabai herself sentenced him to death, and he was crushed beneath the foot of an elephant.
It sounds impossible: How could a mother do this? Let alone one so revered for her compassion and piety. But the story is cited by some as evidence of her divine wisdom and the depth of her caring for the people of her kingdom. She was not going to let them be ruled by a madman.
— Ahilyabai Holkar: the people’s ruler, the people’s goddess
December 10, 2018 (Aliza)