Tamasic austerity carries two poisons. The first: a person torments their own body out of sheer foolish stubbornness, without any understanding of what they are doing or why. The second: the austerity is performed with the specific aim of harming or destroying someone else.
Stubbornness without wisdom is dangerous territory. When someone starves themselves past all reason, not out of devotion but out of blind obstinacy, the practice does more harm than good. The body suffers, the mind darkens, and no purification happens.
Even worse is austerity aimed at another person's downfall. The Puranas contain accounts of powerful beings who undertook extreme penances solely to gain the power to crush their enemies. That kind of tapas, no matter how severe, is tamasic at its core.