The affix ṭhak comes after a word in the Accusative -2nd case in construction when the sense is 'he aids or protects that'.,
Thus समाजं रक्षति = सामाजिकः 'a spectator', lit. 'who aids an assembly by his presence'. So also सांनिवेशिकः ॥,
रक्षति
Adhyāya 4 · Pāda 4 · Rule 33