The mechanisms underlying the generation of magnetic background in magnetography are analyzed from the standpoint of random collisions between toner particles during development. Particles are supposed to be either already captive in non-image area, or still free in the developer bed. The outcome of collisions is determined by comparing the magnetostatic energy of the former type of particles, with the kinetic energy of the latter type. The critical process speed, at which background is minimum, depends on the imaging media permeability and the toner magnetic characteristics. The rate at which background decreases for sub-critical speeds is related to the particle size distribution and the number of collisions expected at each development position. Quantitative results from the model are shown to be in fair agreement with experimental data.