An unlikely event is likely to happen because there are so many unlikely events that could happen. Per Bak is one of the founding fathers of the science of self-organised critical systems . Although large events are comparatively rare, events can and do happen on all scales, with no different mechanism needed to explain the rare large events than that which explains the smaller, more common ones. Contingency is irrelevant. Small freak events can never have dramatic consequences.
Chaos theory shows how simple systems can have unpredictable behaviour.
Chaotic systems have no memory of the past and cannot evolve. The complex state is at the border between predictable periodic behaviour and unpredictable chaos. In short, chaos theory cannot explain complexity. Power laws, characteristic of self-organising critical systems, have an interesting property. Earthquakes are clustered in time, not periodic.