"The Decisive Moment" was one of a long list of suggested titles for HCB's book. The original, French title is "Images a la Sauvette" which translates roughly as "Images on the Run (or in Haste)." Actually, HCB's preferred initial title was "A Pas de Loup" (Tiptoeing). The U.S. publisher did not like that title in English and asked HCB fro some alternatives. "The Decisive Moment" was chosen, but I don't know that it was really HCB's philosophy or hard and fast rule. He just got stuck with it as a consequence.
The term "l'instant décisif" comes from the excerpt from the Cardinal de Retz's
Mémoires that Henri Cartier-Bresson put as an epigraph to his preface to
Images à la sauvette. It reads:
"Il n'y a rien dans le monde qui n'ait son instant décisif, et le chef-d'oeuvre de la bonne conduite est de connaître et prendre ce moment."
HCB, I believe, only kept the first part of the sentence [Edit: after checking, indeed, only the first part was used]. The
Mémoires were published in 1717, so the original intent or meaning of the expression had little to do with photography.