It's lighting. Leaves generate energy through photosynthesis, but also cost energy to run. If there's plenty of light, then the leaf's net energy balance is positive - it generates more than it uses - so the plant keeps it. If light levels drop, then the leaf costs more than it produces - its energy balance is negative - so the plant drops it. In a stand of plants, the light is obviously diminished inside the stand, so the plants inside the stand drop their lower leaves. In slightly lower light conditions, the shading of the single plant's upper leaves may be enough to cause the plant to shed the lower ones. This is one reason why I spread my four tubes front to back across the top of the tank; it increases the amount of light coming in at an angle and missing the upper leaves. Still lose lower leaves within the stand, however, but as I say, even Bacopa does this. Not seen it in Cabomba, but this might be because in nature it tends to grow like Hornwort along the water surface rather than vertically, so it doesn't really have "lower leaves", and therefore has not evolved/been given by God* this energy optimising mechanism.
*delete as appropriate to avoid argument.