In quantum mechanics, the behavior of particles and systems is described by probabilistic principles rather than deterministic ones. This means that at the fundamental level, certain events or outcomes cannot be predicted with certainty but only with a certain probability.
According to the Copenhagen interpretation, one of the most widely accepted interpretations of quantum mechanics, the act of measurement or observation collapses the wavefunction, determining the state or properties of a particle. The specific outcome of a measurement is probabilistic, and different outcomes can occur with certain probabilities.
So, in quantum mechanics, events or outcomes happen due to a combination of probabilities and interactions between particles and systems. While we can calculate the probabilities of different outcomes, we cannot attribute them to specific causes in the classical sense. Quantum mechanics introduces inherent randomness and uncertainty into the behavior of particles, challenging the notion of deterministic causality that we are familiar with in classical physics.
It's worth noting that there are alternative interpretations of quantum mechanics, such as the many-worlds interpretation or the pilot-wave theory, which propose different ways to explain the behavior of particles and the existence of definite outcomes. However, these interpretations are subject to ongoing debate and are not universally accepted within the scientific community.