All plant material are basically consisting of Cellulose Fiber, Hemicelluloses and Lignin, which bind cellulose fibers together. Pulping is nothing but breaking/removing lignin to separate fibers. Lignin is physically and chemically weaker than cellulose fiber. Hence when a physical force or chemical is applied to plant (wood, grass, straw, rag etc.), lignin breaks down faster than cellulose. Heat also weaken lignin faster than cellulose fiber.