Systematic calculations are performed of rate constants of total ionization in collisions of two alkali-metal atoms, one of which is in the ground state, and the other Rydberg atom. It is shown that the first-order perturbation theory with respect to interatomic interaction is not applicable to such calculations. The large discrepancy between theoretical predictions and experimental data obtained for low Rydberg states was found in three typical schemes of experiments (a gas cell, single beam, and crossed beams). To improve the existing model of the collision ionization process, it is suggested to introduce the additional Arnold diffusion mechanism of a random walk of a Rydberg electron over a discrete energy spectrum.