Karnatakabhashabhushana 3.15.2

3.15.2

akārāntāt śabdāt napuṁsakaliṅgē vivakṣitē svarādau vibhaktigaṇē parē dakārāgamō bhavati.

After a word ending in a, when the neuter gender is to be expressed and a case inflection group beginning with a vowel follows, the augment d occurs.