When the original Band first came out, some people complained that it was counting steps while they were driving. The only time I experienced this issue was when I used my steering wheel as a drum to play along with music from my car stereo. One of the users in these forums explained that it is the repetitive motion that triggers the step counter. I believe they said 12 consecutive movements is the magic number to start the counter but I can't find that documented from Microsoft anywhere.
I think it also has to do something with sensing vibrations along with the swinging motion - vibrations that a foot impacting the ground could cause, but that could also be simulated when you beat the drum. I can't get my Band 2 to count steps just by swinging my arm back and forth as if I were walking, but I have experienced it counting steps while carrying objects in my arms that would prevent my normal arm swing while walking.