There is really not a single event that caused the Second Continental Congress to convene. The Second Continental Congress was scheduled before the First Continental Congress adjourned. The delegates agreed at that time (fall of 1774) to meet again on May 10, 1775. Thus, the immediate cause of the Second Continental Congress was the fact that the First Continental Congress had scheduled it.
However, I doubt that this is the answer that your instructor expects to see. I expect that you are supposed to say why the colonists decided to call a Congress at all. The best answer to this is that the Congress was called because of the Intolerable Acts (called the Coercive Acts by the British). These were laws that were passed by the British government to punish the city of Boston and the colony of Massachusetts for the destruction of property in the Boston Tea Party. The colonists felt that these laws were unjust and intolerable and they called a congress to discuss what to do about them. I would say, then, that the Intolerable Acts were the cause of the Second Continental Congress.