Two Boeing 737-800 aircraft were involved in a collision on the ground at Toronto-Pearson International Airport, Canada. WestJet flight 2425, a Boeing 737-800 registered C-FDMB, landed on runway 24R at 17:39 hours local time following a flight from Canc£n, Mexico. The flight taxied to Terminal 3 and was stopped with the engines and APU running, on Taxi-lane 2 on the north side of Pier B. The flight crew were waiting for ground personnel to marshal the aircraft into position at Gate B12. The WestJet aircraft was positioned directly behind and perpendicular to a Sunwing Airlines Boeing 737-800 (C-FPRP), which was parked at Gate B13. The Sunwing aircraft was being prepared to be towed to another location at the airport. A Sunwing maintenance technician was in the cockpit of C-FPRP, and the aircraft’s APU was running. Two ground personnel were in the cab of the tow vehicle. The tow vehicle was informed by the North Apron controller to pushback at their discretion. The Sunwing aircraft (under tow) began to move back and the APU contacted the right wing of the stationary WestJet aircraft. A fire rapidly ensued. The WestJet flight crew immediately commenced the required steps for an evacuation, and an evacuation was ordered shortly after. The tow vehicle operator pulled the Sunwing aircraft back towards the gate to separate the two aircraft. The slides on the WestJet aircraft were utilized on doors L1, R1 and L2, along with the over-wing exits on the left side; all passengers evacuated safely. The maintenance technician aboard the Sunwing aircraft exited the left cockpit window using the emergency rope and received minor injuries. ARFF arrived at the scene, and extinguished the fire. The Sunwing aircraft sustained substantial damage. Summary: In this accident, the pushback was conducted without the use of wing walkers, which is not in accordance with Swissport, Sunwing, and GTAA requirements. The investigation also determined that wing walkers were normally used by Swissport only when pushing back aircraft with passengers on board. In addition, the GTAA apron radio officer used phraseology that was not consistent with GTAA AMU procedures. WestJet’s pre-flight safety briefings do not inform passengers to leave behind carry-on baggage in the event of an evacuation. During this occurrence, several passengers retrieved their carry-on baggage, despite the fact that FAs repeatedly provided specific instructions to the contrary. These passenger actions, in combination with the lack of emergency lighting, delayed the evacuation process.