In To Kill a Mockingbird, why does the Old Sarum bunch come to the jail?

In the South back in the days of the Tom Robinson trial it was not uncommon for justice to be taken into the hands of an angry mob resulting in the hanging of a poor black man or boy.  There was also very little consequences placed on the persons who would commit such a crime.  The mob that gathered at the jail had this intention when they first arrived.

Atticus was well aware that the people would probably behave in this manner and that was why he was at the jail that night.  Since the sheriff was not able to be there someone had to protect Tom from the mob.  In the end, it was Scout's words that dismantled the mob.