Well, sometimes companies feel their current logo is too out of date and they want to change it.
Or sometimes companies rebrand completely, sometimes because of a shift of focus. (A company I know about shifted from discrete software to B2B software and changed its branding in the process)
A legitimate reason is when a company or organisation changes significantly. Wikipedia changes remarkably over time, but does it incrementally. Logo change is neither sensible or required.
Is there every a "reason" to create new logo's or icons? Other than someone feels a new for a new or fresh look...