Blame Random Events and Covariance
Random events aren’t predictable but also occur in clusters. Think about it. By definition, you can’t predict a random event. “They” always say aircraft crashes happen in threes, or bad things happen together, but there is no other way. If we could predict an airplane crash we wouldn’t fly on that day. The more familiar reference to random events is known as ‘Murphys Law’ — anything that can go wrong, will go wrong.
If the event is not deterministic, or predictable, then it will not fall within a known interval of time. Therefore, some intervals between events will be longer, some shorter. The shorter intervals are referred to as ‘clusters’.
The second, less familiar mathematical principal is a phenomenon called covariance. Typically considered a vary sophisticated mathematical concept with corresponding complex solutions, just having an awareness of it is enough to help you manage the impact.