Well, reason has two meanings, "to think something through", and a noun "the cause" something happened which could be animate or inanimate. Motive (the noun) is "the cause" of something happening but requires there to be a guiding hand, so "I wanted to win, was my motive during the game" but you could not say "the cliff was crumbling and so that is the motive it fell down" as the cliff has no brain.