Brand names that changed in everyday usage

[PastTense]

Exxon used to be Esso (and some other names):

History of ExxonMobil

And older thread:

Why do companies change their names?

I have noticed lately that a lot of companies keep changing their names. Audio King is now Ultimate Electronics, Verizon is a relatively new name, Dayton’s decided to be Marshall Field’s, Accenture used to be another name (some accounting firm, I think). Those are the only examples I can think of off the top of my head, but I am curious as to why it seems so frequent lately. Is it because they have no clearly defined corporate identity, or is there usually a legal reason behind it?