Zero-electron-kinetic-enegy photoelectron spectroscopy ("ZEKE-PES") is based on the pulsed field ionization of long lived Rydberg states (ZEKE states); it is generally accepted that ZEKE states have large angular momentum l, which quenches electron-core interactions, but how they acquire t remains a matter of dispute. We show that {nl}-->{nl’} ion-Rydberg collisions are a viable and prominent mechanism for the excitation of large-l Rydberg states. We elucidate the dynamics by an exactly solvable classical model which provides a transparent and intuitive picture of the excitation of high-l states. By a geometric interpretation of the dynamics we are able to predict for which values of the impact parameter and reduced velocity of the incoming ion a change of the angular momentum of the state becomes possible. We pay particular attention to the influence of the quantum defect, delta(l) on the {nl}-->{nl}’ cross section and demonstrate that, for small initial angular momenta, delta(l) is itself a major contributor to the experimentally observed scaling of the cross section as similar to n(5). This classical-quantum defect model is not only able to explain the dependence of the experimentally measured fractional population of high-d states on the velocity of the incoming ion, but leads to the prediction that dipole-forbidden transitions are likely to dominate l transitions induced by ion-Rydberg collisions under ZEKE-PES conditions.