I've seen this phenomenon a lot of times but I can't give it a physically explanation. Someone has an idea?
The wheels are not spinning while the plane is in the air. At the moment that they first make contact with the ground, they skid a bit before they start rolling at a rotational rate consistent with the ground-speed of the plane. That skid generates heat and burns off a little rubber. (This also accounts for the black smears on the runway at the point where planes typically first touch down.)