mind of (one's) own
The propensity or ability to think, act, or form opinions without outside influence. It is often used (humorously) to describe an inanimate object that cannot be controlled by its owner. I tried to tell him that wasn't the best decision to make under those circumstances, but you know he has a mind of his own, and he did it anyway. I swear, my vacuum has a mind of its own! It turns on in the middle of the night when we're all asleep!