Poor building process It's possible for the mason to be in a haste, or build a wall when conditions are not proper. He might not prepare adequately for the conditions he is working in. He may throw lots of junk in the trench by the walls as it's being built, and that can create issues later on. He may put the wrong size sill plate on the wall, creating lateral pressure on the wall, or he might backfill too soon after building the wall, while the concrete is still curing. Settling Footers (the things your foundation sits on) may be built on the loose material. It might expand or contract over time because of the weight on it, or you may be in an area with soils that move. In any case, settling results from the shifting. What you'll see is cracks that develop from that settling. Deterioration Like walls and roof, the foundation of a house should be maintained periodically. The coatings put on the walls can break down over time, allowing water to come in contact with the concrete. Once that happens, that water begins to affect the strength of the concrete. At that point, you might begin to see discolorations, water, cracks, or even bowing of the walls. Age Time breaks down just about everything. The fact is, basement structures probably only last a hundred years or so, and then they need to be replaced and that if no other outside factors are involved. Add settling, shifting, expansive soils and outside pressures, and you can cut that number down considerably. Shrinkage comes from the curing of the concrete or mortar. As it shrinks, cracks develop, usually along mortar lines, but sometimes in the form of vertical cracks. Once there is a crack it is weaker than the surrounding wall, and that leads to the possibility that pressures from outside will begin to bow that wall.