The affix ghañ comes after the root grah 'to seize' in composition with ava and ni when malediction is meant.,
Though इनुण् is immediately, near, we take the anuvritti of घञ् 3.3.16, because we see that its anuvritti is possible.
The word आक्रोश means 'cursing'. As अवग्राहो हन्त ते वृषलभूयात् 'cursed be thou, O! sinner'. So also निग्राहस्ते भूयात् ।
Why 'when meaning cursing'? Observe अवग्रहः पदस्य; निग्रहश्चोरस्य ।,