Some of those reasons are positive (two organizations have merged or a company has significantly expanded its offering), while others are less rosy (the current brand has been tainted in some way or has become outdated).
Regardless of the reason for rebranding, there are right and wrong ways to go about it.
In situations when a brand has been firmly established yet is simply outdated or needs to be refreshed due to the addition of new products or services, tweaking is required, rather than a full-blown rebrand.
In these cases, you don't want to eliminate the brand value that's been developed over the years, but merely make subtle changes to update it or make it representative of an expanded offering.
The image used for years on its products--an African-American "blue collar" woman--was simply out of step with today's mores.
Corporate mergers will often result in complete rebrands.