Water naturally ionizes to some extent, depending on physical conditions such as temperature and pressure. See the following pages for an introduction to this topic:
Other fluids may be subject to a similar process. When such fluids are mixed, or when other compounds are added to that solvent, these additives may need to reach their own ionization equilibrium to adjust to the new environment, since they are now surrounded by an ionized material. Of course, each material type reacts in its own way, and some may remain unaffected altogether.