زَيْدٌ قَامَ أَبُوهُ
Zayd's father stood up.
The subject of «قَامَ» is «أَبُوهُ»; the attached pronoun refers back to Zayd. The entire verbal clause is the predicate of Zayd, but the act of standing is not attributed directly to Zayd. It is attributed to his father, whose relation to Zayd makes the clause relevant to the main subject.
This indirect linkage explains the term *sababī*. «قَامَ أَبُوهُ» is a verbal sentence, whereas «قَائِمٌ» could predicate standing directly of Zayd. Conflating these would change the structure of the attributed action.