It's basically the 80/20 rule and it's found in many areas of life. For instance, for many businesses 20% of their customer base accounts for 80% of their sales. Similarly, a majority of crimes are committed by a small group of people.

Malcolm Gladwell wrote about this phenomenon in a few of his books. STD outbreaks can usually be traced to promiscuous individuals that sleep with other promiscuous individuals that sleep with and maybe with individuals that aren't necessarily as promiscuous. Promiscuous individuals also tend to engage in other risky behaviors, such as intravenous drug use. These people tend to operate in similar areas and thus, the STDs are able to spread quicker to this small group until it's an epidemic. But it is a small group that is responsible for all of this epidemic. If this small group is persuaded to have safe sex, future outbreaks can be prevented just simply because this risky group has been removed from the equation.

This is also applicable in crime. In the shitty part of town, most crime can be traced back to small groups. They may only be 20% of the overall pool of criminals, but they account for a disproportionate amount of the crimes committed. This is your group of repeat offenders that treat prison gates like a revolving door. My family has an extensive history in law enforcement and they can all attest to this theory, it's why they become so familiar with repeat offenders.