If I understand it correctly, wood has the fiberous material bonded by resins. The heat softens the resins, allowing the long fiberous material to slip during a bend. One side of the stock is in compression and the other side is in tension during a bend. Beyond that, I dunno what else might be going on in the wood during a bend. It stands to reason that various wood species would have different resin and fiber properties. Possibly one type of wood has different "melting" temperature vs another species of wood.