What structural instability causes the transition from radiation to convection near the edge of the zone?

The temperature gradient becoming too steep over a small distance.

The boundary between the radiative zone and the convection zone is not static; it shifts over stellar lifetimes as the core composition changes, primarily through hydrogen burning into helium. This change in fuel affects the thermal structure. When the temperature drops too rapidly across a small radial distance—meaning the temperature gradient becomes too steep—the plasma at that location becomes unstable to convective forces. At this transition point, the physical mixing of the plasma (convection) becomes a much more energetically favorable and faster method for transporting heat than the slow, iterative movement of photons via radiation.