Why bearing pressure under a footing isn't uniform: eccentricity and the middle-third rule
A footing loaded exactly through its centroid presses on the soil evenly. Almost no real footing is — because the load acting on it is rarely a pure vertical force alone, and a retaining wall's base is essentially never loaded that simply.
Whenever a footing carries a moment in addition to a vertical load — an eccentrically framed column, or (essentially always) a retaining wall base reacting to lateral earth pressure — that combination is equivalent to the same vertical load acting off-center, at an eccentricity e = M / V from the footing's centroid. An off-center load doesn't press on the soil evenly: it produces a trapezoidal pressure distribution instead, higher on the side the load leans toward and lower on the opposite side, rather than the single uniform value a perfectly centered load would give.
The middle-third rule (kern) and what happens past it
For a rectangular base, as long as the eccentricity stays within the middle third of the base width (e ≤ B/6 — the base's "kern"), the whole base stays in contact with the soil, with pressure varying linearly between a higher and a lower value, both still compressive. Push the eccentricity past B/6, and the linear-elastic formula would predict tension at the far edge — soil can't actually take tension, so in reality part of the base lifts off and the remaining contact area redistributes the same load over less area, concentrating pressure on the loaded side. Exceeding the middle third isn't automatically a failure, but it's generally avoided in practice for exactly that reason: a smaller effective contact area means a higher peak pressure for the same total load, and a foundation further from the simple, well-behaved linear case the design was probably checked against. Foundation Design and Retaining Wall Design both compute the resulting maximum (and minimum) bearing pressure directly from the actual eccentricity, rather than assuming a uniform pressure that wouldn't reflect how an eccentrically loaded base actually behaves (see the related tools below).