Finishes with drying oils like boiled linseed or tung are known for doing this. They self heat as they polymerize so a pile of them could heat up to the ignition temperature because the heat won’t escape. Placing them in a bucket with plenty of water prevents this by the thermal mass of the water and blocking the oxidation of the oil. Laying them flat away from each other also works by preventing the heat from building up.

Putting a flame or heat gun next to the finish isn’t such a great idea either. There’s often flammable chemicals in finishes beyond just the oils.