If the shifter wasn't firmly seated in the neutral detent it is possible a sharp bump caused it to jump into the next position, in this case drive.
I just had this happen with the ignition switch on our toad. Though it was in the off/unlocked position, somehow it popped into the off/locked position locking the steering. Fortunately for us it happened on a gravel drive so no damage beside my ego and some disturbed gravel.
I now double check all shifter and key positions. Let the shifter "pop" into neutral and rock it slightly to ensure it is fully in the detent.