It is the tires that are smoking as they slip. Remember from many moons ago difficulty in pushbacks. Problem was that B707 and DC-8 didn't have APU's. The generators on the back of the Hough tugs were finicky and the long length of the ground power cable a pain so, to avoid a pushback with no lights in the cabin they would often start one engine on the gate prior to pushback. The Hough's at LASI NY were fitted with carbide studded tires for winter. Had one of these dig some interesting cavities into the cement of the tarmac as the wheels spun. Port Authority was not amused. Easter Air Lines did powerbacks with engines in later years. Machinist Union was not happy as it eliminated need for tug and driver which of course was reason for doing it.
Was up in YYZ once in a major storm. The B747 on the gate next to us had been deiced and they tried to pushback but no hope of traction even with chains. They requested a little reverse thrust, flight crew a little too enthusiastic and the blast blew windows, bag carts, etc.
Even back in the days of Connies and DC-6 pulling a plane into the hangar with a CT-120 tug could be difficult. The hangar floors are smooth, not like the tarmac or ramp outside. Even a wet floor with a little dripped oil could defeat it. Have had whole crew of mechanics get on the tug for weight and Zorball sawdust liberally spread.