Physicit purpots to explain everything using three concepts; the innitial conditions, the boundary conditions and the laws. It is in the boundary and innitial conditions that some want to artribute it to chance. So we must get it clear what this word rifers to.
As usual, dictionaries offers lots of definitions to words. Some are hard or even impossible to reconcile. However from the context, we can guess that when they artribute 'chance' to a natural phenomenon, they mean 'happening with no intent'. So by saying 'it happened by chance', he is actualy not talking of what happened at all!! He is talking of what it did not happen, i.e 'intent did not happen'. So what have we understood?
Because the 'intent' is in the definition and they are using 'chance' like a mantra, you might think that they have a good grip of what intent is. But they don't! Their entire approach is to attempt to completly remove intend from nature so that even human has non! But this make the very concept 'chance' meaningless!