There are several users that report they get steps while driving. I checked that during a two hour drive and I didn't get any steps at all. I guess it depends on what you are doing with your hands during the drive, the surface of the road, vibrations of the steering wheel and maybe some other factors that could make the watch start to count steps.
What type of car? Modern or older? How was the surface of the road?
As Andy wrote, and if you do not find the way how to avoid it happening again. you can always start some kind of Activity that turns off the steps counting. For example the Indoor Cycling is a good choice, because it does not use GPS, and hence it saves the battery consumption. You can discard the activity at the end.
