The culprit of the smoke: birds colliding with the plane's nose and engine. The pilots quickly returned to Cuba and the passengers used slides to evacuate.
According to a statement from Southwest to CNN:
"Southwest flight #3923 departing Havana, Cuba, for Fort Lauderdale, Florida, on Sunday morning reportedly experienced bird strikes to an engine and the aircraft's nose shortly after takeoff … The pilots safely returned to Havana where customers evacuated the aircraft via slides due to the observation of smoke in the cabin.
"We commend the swift, professional actions of our pilots and flight attendants in responding to this event. We apologize to our customers for the inconvenience and have reached out to address their needs and offer support."
No injuries to passengers or crew were reported.