What led British soldiers to fire on Boston townspeople on March 5, 1770 Parliament had repealed the Townshend Acts.
The townspeople threw objects at the soldiers.
British soldiers were put on trial for murder.
The soldiers had begun to enforce the Quartering Act.

The townspeople threw object at the soldiers.

March 5, 1770 was the day of the Boston Massacre, where a mob of American colonists gather around the Boston Customs House to protest the occupation of soldiers in Boston. So, they began throwing snowballs, stones, and sticks at the Soldiers on duty to protect the Building. As the mob got bigger, the soldier became more worry about their safety and thus open fired in the Boston Townspeople.