Mentioning Zayd as the opening subject creates an expectation: the audience anticipates that something is about to be predicated of him.
هُوَ أَنَّ الْمُبْتَدَأَ لِكَوْنِهِ مُسْنَدًا إِلَيْهِ يَسْتَدْعِي أَنْ يُسْنَدَ إِلَيْهِ شَيْءٌ
A mubtadaʾ, by functioning as a subject, calls for something to be attributed to it.
The listener is prepared for an assertion concerning Zayd. That preparation contributes to strengthened attribution, which must also be understood through the returning pronoun.
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When the Predicate Is a Clause: Strengthened Attribution, Causal Predication, and Adverbial Phrases
Mentioning Zayd as the opening subject creates an expectation: the audience anticipates that something is about to be predicated of him.
هُوَ أَنَّ الْمُبْتَدَأَ لِكَوْنِهِ مُسْنَدًا إِلَيْهِ يَسْتَدْعِي أَنْ يُسْنَدَ إِلَيْهِ شَيْءٌ
A mubtadaʾ, by functioning as a subject, calls for something to be attributed to it.
The listener is prepared for an assertion concerning Zayd. That preparation contributes to strengthened attribution, which must also be understood through the returning pronoun.