As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING PUSH BACK, SMOKE WAS OBSERVED IN THE CABIN AREA WHILE ENGINES WERE BEING STARTED. THE AIRCRAFT RETURNED TO THE GATE. ENGINE STARTS WERE CARRIED OUT AT THE GATE AND THE SMOKE WAS NO LONGER OBSERVED IN THE CABIN. DE-ICING FLUID WAS SUSPECTED. NO FURTHER INCIDENT DURING SUBSEQUENT START.