The trigger describes why you are communicating about the context to this audience at this particular point in time.
It flows naturally from the context to form a narrative flow that then leads tightly to the central question that the document aims to answer. Modifying the trigger will significantly alter the question and the nature of the whole story.
The trigger varies in length from half a sentence for many documents to a paragraph in longer pieces of communication. It may be longer still if you are writing a book.