I believe "set in motion," "trigger," and "propel" carry shades of meaning based on their original, literal usage.
A gun has a "trigger," and is "triggered." It refers to a small force that triggers a single, loud, violent explosion. It also has a characteristic of suddenness. You gradually increase the force on the trigger and suddenly the gun fires.
A machine is "set in motion" by starting it. It carries a feeling of some mechanical complexity. The motion is steady.
"Propel" makes me think of a ship and its oars, or its propellor, or an airplane propellor. It refers to something that is already in motion. It makes me think of the continuing source of energy needed to maintain that motion.