When historians study the past, they try to explain why events happened the way they did by asking questions such as: Why did World War II happen? How did it start? Asking these questions help identity the various factors that caused the event, including both the immediate factors and the underlying factors. 

Immediate causal factors are directly linked to (that is, causing) the event occuring. They typically take place just prior to the event in question. 

Underlying factors represent broader underlying elements, practices, or beliefs that are not directly tied to a single event but contribute to it.

For example, the immediate causes of a bushfire can be lightning  combined with high temperature and dry bush. Underlying causal factors could be broad human activities, deforestation, and global warming. 

When historians study the causes of an event, they try to determine the importance of each cause as some factors are more instrumental than others in bringing about and shaping an event (e.g., factor directly linked to the event, factor affected the intensity/magnitude of the event).

In looking at the importance of causes, it becomes possible to consider (a) whether the event could have happened differently or not (for example, if SS City of Benares had been positioned elswhere in the convoy or identified as a evacuee ship), and (b) what is the agency or responsability of individuals or groups in causing a change (what role and power persons/groups have in the relation to the event). 

 See the following documents to understand more about the concept of causation in history (from the Project of Historical Thinking)