A comprehensive theoretical analysis of photoinduced forces in an illuminated nanojunction, formed between an atomic force microscopy tip and a sample, is presented. The formalism is valid within the dipolar approximation and includes multiple scattering effects between the tip, sample, and a planar substrate through a dyadic Green's function approach. This physically intuitive description allows a detailed look at the quantitative contribution of multiple scattering effects to the measured photoinduced force, effects that are typically unaccounted for in simpler analytical models. Our findings show that the presence of the planar substrate and anisotropy of the tip have a substantial effect on the magnitude and the spectral response of the photoinduced force exerted on the tip. Unlike previous models, our calculations predict photoinduced forces that are within range of experimentally measured values in photoinduced force microscopy (PiFM) experiments.