When a stranger is yelling at a trans person about their biological sex, it's done to inflict emotional harm on the trans person. They transitioned away from that gender to reduce harm (that is the definition of a trans person), and the stranger is intentionally trying to bring that harm back.
Imagine shouting at a woman about her "biological breast size" because she is wearing a push-up bra, or had surgery to enlarge or reduce her breast size. Would that seem like a normal, harm-free way to interact with another person you don't know? Obviously not.