In everyday use, “chance” means an outcome wasn’t deliberately arranged. In the statistical sense this site uses, calling something due to chance means it is well described by a probability model with no special mechanism added — not that it is uncaused in some deeper sense. A coin landing heads has plenty of physical causes; it is still modelled as “chance” because those causes are treated as unpredictable background detail rather than something worth tracking individually.
See Probability is a model, not a verdict for why this framing matters for coincidences specifically.