Was the car locked when you had it parked? If the button/clip on the steering wheel was broken the wiring could have caused the radio to change stations or its possible you did just knock it (the radio) But the broken ones are the head scratcher. Could you have possibly caught them with your brollie or bag or something that broke it? If not I would say kids or someone has been in your car & has attempted to get your radio out but is stupid & thought they needed the buttons off the steering wheel. OR they where disturbed & took off before being able to do any real damage/take the car.

But if the car was locked & there is no sign of forced entry then I would think you have managed to break it somehow when either getting out last or when you got in out of the rain in a hurry?