Okay, I’ll bite. From the Wikipedia page:
One part of the controversy concerned the parts of the book which dealt with racial group differences on IQ and the consequences of this. The authors were reported throughout the popular press as arguing that these IQ differences are genetic, and they did indeed write in chapter 13: “It seems highly likely to us that both genes and the environment have something to do with racial differences.” The introduction to the chapter more cautiously states, “The debate about whether and how much genes and environment have to do with ethnic differences remains unresolved.”
Whether it was their intention or not, it was clearly controversial, and many critics accused the book of racist undertones/motivations.