The study didn’t examine why

The study uses data from a US sample of 7,897 youths born between 1980 and 1984 who were interviewed annually between 1997 and 2011. The concentration of immigrants in each respondent’s county was estimated from US census data. Various time-varying (e.g., age, school dropout, concentrated disadvantage of the neighbourhood) and time-invariant (e.g., sex, race, parental education) factors were controlled for.

Within individuals, increases in immigrant concentration in the area in which they live reduces the likelihood of arrest. Furthermore, the effect on arrest of the change in the percent foreign-born in a person’s neighbourhood does not depend on whether individuals moved into a county with a different concentration of foreign-born residents, or whether the country itself changed in its proportion of immigrant residents.

During the period of time in which data were collected for this study, there was, generally, an increase in the proportion of foreign-born people in the various counties in which respondents lived. The “crime reduction” effect of a higher concentration of immigrants in a person’s county tended to reduce arrests for other immigrants, their children, and others.

You could speculate a range of reasons - offending could be less of a norm in these communities or there could less opportunities to offend in areas with stronger community bonds or a greater risk of punishment outside the traditional police/justice system. Migrants are also more likely to engage in socializing and exchange of help with neighbors, and consequently their neighboring helps strengthen community links.