The Stalemate Detector is a system which gets triggered under tie-breaking situations. The Detector checks for certain match-ups and grants one of the remaining roles the victory, ending the game prematurely to prevent draws from taking place.
This was implemented to save everyone time from stalemates and common draws, which the additional night/day cycles would be needed to let the game end in a draw.
Situations the Stalemate Detector covers can be found on the Town of Salem Wiki.