The affix ṭac comes after the word brahmaṇa in a tat-puruṣa samāsa when the compound denotes a person belonging to a certain country.,
The word जानपदः = जनपदेषु भवः 'belonging to a country'. This affix comes after that Tatpurusha the first member of which is a word denoting a country. Thus सुराष्ट्रेषु ब्रह्मा = सुराष्ट्रब्रह्मः, and अवन्तिब्रह्मः \a Brahmana of Surashtra or Avanti\. This is an example of Locative Tatpurusha compound.
Why do we say \when referring to the name of a country\? Observe देवब्रह्मा नारदः 'Narada the bard of the gods'.,