Masonry chimneys stand in the weather all their lives and over time without weather proofing they begin to deteriorate, slowly at first then they seem to reach a tipping point and then they go down hill exponentially fast. The chimney crowns soften and crack allowing water in to the venting system which degrades and deteriorates the chimney from the inside out. The mortar joints in the chimney shell get washed out, loosening bricks leading to structural instability. Chimneys with out a cap take on rain that pools in the smoke shelf, seeping in to the fire bricks of the back wall of the fire box, softening and cracking them.
Water can also be wicked up from the ground affecting the chimney footing and firebox deteriorating the chimney structure from the inside out. When you see a chimney with loose bricks, the top is missing bricks, crumbling or has severely deteriorated mortar joints is well past time to call a chimney sweep.