Every once in a while my engine will start upon starting the car. I assumed it is programmed to periodically do that to keep the fluids circulated and the engine ready to go if called upon for stronger acceleration. Otherwise, it seems only kicks in when battery is low, I put it into Sport mode instead of my usual Eco mode, or I press hard on the accelerator to quickly accelerate.
What mode do you have it set in? Perhaps Sport mode does that to warm up the engine and be ready for prompt performance when called upon?