When does the second sentence in the student-and-bus example function as istiʾnāf?

Supplementary Practice 3: Rhetorical Judgment, Contextual Evidence and Combined Devices

• When the second clause only repeats the first judgment
• When the first report naturally raises a question about the cause of the delay
• When the second clause is offered as a distinct report
• When the two verbs merely share the same tense