Assuming you Creep Mode set to OFF, I think the likely explanation for what you describe is that the Tesla drivetrain does not behave the same way an ICE car transmission does and your car moved forward because it was on a slope but you weren’t aware of it. Let me explain:
In a Tesla, when you go from Park to Reverse or Drive, with Creep Mode OFF, if your foot is not on the brake and you are not applying sufficient pressure to the accelerator pedal to cause the car to move the car will behave as if it is in neutral. That means that if the road (or your driveway) has enough slope the car will start to roll.
If Creep Mode is set to ON in that scenario the car will move backwards unless the slope is to much for the car to overcome with no pressure on the accelerator pedal.
I am willing to bet that when you went from park to reverse your foot was not on the brake (which is not safe driving practice) and you were not applying pressure to the accelerator AND you were not on level ground.
So far you have not told us if Creep was on or off at the time of the incident you describe nor have you told us if you were on level ground or on a slope.