The old one was based on the Golden Gate Bridge. The name itself refers to the city of San Francisco, although everybody who lives there know that you should NEVER shorten the name to Cisco (or San Fran for that matter).
The new logo still vaguely reminds of the Golden Gate Bridge. It’s just different.
Perhaps Cisco thought that it was falling behind after Intel changed its logo late last year. But everybody knows that these exercises really are the product of a dark job security conspiracy by graphic designers.