Why do landing gear tires smoke upon touchdown?
Because the tires are momentarily skidding on the pavement as they rapidly spin up from standstill to the jet’s touchdown speed.  It’s just a puff of burnt rubber but the tire tread itself becomes quite warm, reaching 600-700°F for an instant.  It’s also the reason a pilot does not ride the brakes upon touchdown as this could cause a blowout as the tire is ground up rapidly.