فَإِنْ كَانَ ذُو الْحَالِ نَكِرَةً، يَجِبُ تَقْدِيمُ الْحَالِ عَلَيْهِ، نَحْوُ: «جَاءَنِي رَاكِبًا رَجُلٌ»؛ لِئَلَّا تَلْتَبِسَ بِالصِّفَةِ فِي حَالَةِ النَّصْبِ فِي مِثْلِ قَوْلِكَ: «رَأَيْتُ رَجُلًا رَاكِبًا».
Translation: If the referent is indefinite, the author requires placing the ḥāl before it in this construction: “A man came to me riding,” to avoid confusion with an adjective as in “I saw a riding man.”
The explicit reason is avoiding confusion with an attributive adjective. Word order helps distinguish the two analyses.