«كَوْنِ الْجُمْلَةِ الثَّانِيَةِ جَوَابًا لِسُؤَالٍ اقْتَضَتْهُ الْأُولَى»
The second clause answers a question prompted by the first.
The first clause states something that naturally arouses a follow-up question: “What happened next?”, “Why?”, or “What does that mean?” The speaker leaves the question unspoken and moves directly to its answer.
This is explanatory resumption (istiʾnāf bayānī). The latter clause begins afresh because its job is to answer a question that the earlier clause has generated in the listener’s mind.