The Polyurethane Blog has an account of a large house fire that is being blamed on spontaneous combustion of polyurethane foam insulation. My first reaction on reading this was to shout "Boy Scout" or more properly, the initials of "Boy Scout". Polyurethanes are stable polymers that don't catch on fire as any house having wood floors with a urethane finish will verify. Further, the fire had just been put out. Knowing the cause that quickly seems like something appropriate for a news reporter to mention in order to seem really sharp, but a full investigation can take days or even weeks at times.
But a little bit of investigation has forced me to rethink this. Polyurethane foams can indeed spontaneously combust, but only for a short while after they have been applied. The exotherm from the reaction can generate quite a bit of heat and if the insulation is thick enough, that heat can have a difficult time dissipating leading to a fire. Such a scenario has been blamed for 3 house fires in Massachusetts as well.
But it should be pretty clear that spontaneous combustion is only a plausible cause shortly after the polyurethane foam has been sprayed. To blame a fire on it after 24 hours or more would be nonsense. A reacted polyurethane is just that - reacted, and it will not react any further.