It's not automatic

On many important ancient Near Eastern treaties, law codes, or covenants one finds appended an extensive set of curses. These curses function as a kind of “divine safeguard” against breaking the agreement. The general understanding of the ancient Near Eastern religions was that these curses became efficacious by breaching the covenant stipulations in an almost mechanical magical manner. Van der Toorn comments:

one gains the impression that it (the curse) acts quite independently of the relationship between the individual and his gods. The many symbolic actions connected with the oath could, much more than in Israel, also be understood as magical manipulation to render the curses automatically efficacious.

There is, however, hardly any evidence for such a reading in the Old Testament.— Standing in the Breach, page 408