My guess is the rags...all folded up and such....begin to dry and the linseed and spirits generates heat as it does so....kind of like the 5 minute Devcon epoxy (if you have ever used that) as it cures it generates heat and where the heat in the rags is generating hotter as it is folded unto itself....increased heat and fire!
Basically it is a chemical reaction taking place.....the wood is more stable than the rags....relate the two to tinder when you try to start a fire as opposed to a solid block of wood.