I have to agree with SpyderTrap here. Compare year over year, most of the times it is seasonality. So you want to make sure you are comparing apple with apple. For some of my e-commerce clients for example there branded traffic keeps getting higher and higher in October, november, december and then it gets very low. So depending on your products or services it might affect your branded traffic.
Make sure the website is still optimize for there brand names. Some clients during redesign forget that they are not the only one competing for there brand name and you can only see there brand in the domain name. Is your client ranking #1 for all of there brand terms? Is there new competition for there brand name.
I hope it helps cheers!