What were the underlying causes of the Sacco and Vanzetti trial verdict?

There are at least two possible ways to answer this question.  First, the jury might have found Sacco and Vanzetti guilty because the two men were actually guilty.  Second, the jury might have found them guilty because of nativism and fear of radicalism.

Even after all this time, it is not completely clear whether the two men were guilty.  There was certainly evidence that suggested that they did commit the crimes in question.  Many people, conservatives in particular, believe that Sacco and Vanzetti were actually guilty.  This link shows that not only conservatives believe this.  We can therefore say that it is possible that the jury voted to convict the two men because the evidence presented in court convinced them that the men were guilty.

However, I imagine that this question is meant to get you to say that the jury found the men guilty because of nativism and fear of radicalism.  During the late 1800s and early 1900s, there was a very large wave of immigration to the United States.  Many Americans feared the immigrants.  They felt that there were too many immigrants in the United States by the 1920s and they felt that the immigrants had not become sufficiently Americanized.  Because Sacco and Vanzetti were both immigrants, and because neither of them was very assimilated, it is possible to argue that the jury convicted them in part because of nativism.

Many Americans also worried about the immigrants because of what those immigrants believed. There were many immigrants who believed in ideas like socialism and anarchism.  Most Americans thought that those ideas were completely contrary to the American way of life.  They were worried that the ideas were spreading.  They were particularly worried about the fact that anarchists, in particular, seemed to be willing to engage in violent acts against the American system.  For example, anarchists had plotted in 1919 to send bombs to many high government officials.  Sacco and Vanzetti had been friends with many anarchists, including a man who blew himself up outside the home of the attorney general of the US.  The two men themselves held anarchist views.  We can argue that the jury convicted them in part because jurors feared anarchists and were willing to assume that anarchists were guilty of anything even if there was not enough evidence to support the idea.

Thus, it is hard to say exactly what motivated the jury to convict Sacco and Vanzetti.  The men may have been guilty, or the jury might have convicted them because the jury feared immigrants and anarchists.  In fact, both of these statements may be true and the men may have been rightly convicted but for the wrong reasons.