Passengers quoting that there was a fuel smell for a large proportion of the flight and the flight crew made a PA about returning due "not having enough fuel to continue to Milan".
Seems like they had a fuel leak.
The question to be asked then is was the leak from the wing or the engine? No mention of an engine being shut down so perhaps the crew suspected a leak from the wing.
The fire erupted just after landing, possibly caused by leaking fuel being ignited after being sprayed into the hot section during the application of reverse thrust.
Regardless of how it happened I would have initiated an evacuation. I don't understand why they didn't.