Yep, many hay barns have caught fire because the hay was bailed while too wet and it is fermenting so vigorously.

If you make your pile really big you might even get some fusion going.

"If the conditions are right" is doing a hell of a lot more work than 99.99999% of people who say this are letting on. Your really are threading a needle of air supply for combustion vs insulation/containment to build heat. The same goes for spontaneous combustion of rags/paper products.

Anything even slightly warm and moist will "steam" (you're actually seeing condensation but that's beside the point) if it's cold (or at least not warm) and fairly humid out. Nobody's wood chip pile is steaming in the august sun (well it is but you're not seeing it condense but once again, beside the point). And even if it was boiling it'd still be a couple hundo degrees away from anything catching on fire and even then the same containment that let the heat build typically precludes oxygen. If a tree falls in the forest and nobody hears it does it make a sound? If the core of some material pile reaches combustion temperature but there is no oxygen for it is it on fire?