If the mouse is a Razer Naga Epic/Chroma with the buttons on the side. I have seen this issue just about every time I update OS X or get a new Mac. The solution I have used repeatedly is to open up terminal and type in the following:
defaults write com.blizzard.worldofwarcraft disable-expose-fix -bool YES
This fix has been around for a number of years, but always seems to get overlooked… Good luck