The o of doṣ is replaced by ū before the causative ṇi,
Thus दूषयति, दूषयतः, दूषयन्ति ॥ Similar reasons as in गोह, may be given for the root दुष being exhibited as दोष in the sutra. When not followed by णि, we have दोषो वर्त्तते ॥,
दोषो णौ
Adhyāya 6 · Pāda 4 · Rule 90