Electro-osmotic drag (EOD) is usually thought of as a transport mechanism of water inside and through the polymer electrolyte membrane (PEM) in electrochemical devices. However, it has already been shown that the transport of dissolved water in the PEM occurs exclusively via diffusion, provided that the EOD coefficient nd is constant. Consequently, EOD is not a water transport mechanism inside the electrolyte membrane, and this means that its nature is not yet understood. This work proposes a theory that suggests that the root of the EOD is located in the catalyst layers of the electrochemical device where the electric current is generated, and consequently could be linked to one or more of the elementary reaction steps. It is therefore also conceivable that EOD exists at one electrode in an electrochemical device, but not in the other. Moreover, the EOD coefficient nd may depend on the current density as well as the oxidization level of the catalyst. The last consequence, if EOD is linked to an elementary reactions step, it could also be part of the rate-determining elementary step, and this could open pathways to increase the reaction kinetics by finding ways of enhancing the water/hydronium ion transport out of or into the polymer phase.