رَجَعَ الْمُسَافِرُ مُبَكِّرًا. كَانَ قَلِقًا عَلَى أَهْلِهِ.
The traveller returned early. He was worried about his family.
The first statement may raise a natural question: Why did the traveller return early? The next sentence answers that anticipated question, providing a rationale for analysing it as explanatory istiʾnāf.
Not every sentence separated by a full stop is explanatory resumption. The analysis is justified only where a plausible question arises from the first sentence and the next addresses it.