Your assuming the wheel stopped rotating while it was lightly touching the side of the road. Visually you don't see the spokes on the low wheel, but you do on the vertical one, so it was still rotating.
If you think about it there was probably under 1 kg of force between the side of the wheel which has minimal grip and the road surface while he was attached for what 2-5 seconds?. Spin a wheel and try and slow it down from lightly touching the sides like that and it's going to spin for a while.
Further, while on the side the contact point of the wheel had a ~5 feet of leverage to the pivot point of the handlebars and the other contact point was a wheel. So, it would take very little force to get the bike rotating.
PS: I don't have a bike right now, but it should be easy to test if you do.
If you think about it there was probably under 1 kg of force between the side of the wheel which has minimal grip and the road surface while he was attached for what 2-5 seconds?. Spin a wheel and try and slow it down from lightly touching the sides like that and it's going to spin for a while.
Further, while on the side the contact point of the wheel had a ~5 feet of leverage to the pivot point of the handlebars and the other contact point was a wheel. So, it would take very little force to get the bike rotating.
PS: I don't have a bike right now, but it should be easy to test if you do.