Prevent disk image automount when clicking Finder sidebar
I would suggest trying two things. As a disclaimer, I have no idea what could be causing this because it makes no sense. These are just some basic Finder troubleshooting steps:

Suggestion 1:


Open Terminal from /Applications/Utilities
Run the command killall Finder
Log out of your user account and log back in
Try again


Suggestion 2:


Open Finder
Navigate to ~/Library/Preferences
Move the files com.apple.finder.plist and com.apple.finder.plist.lockfile to the Desktop
Log out of your user account and log back in
Try again


If neither of these suggestions work, I don't know what else you could try. Other StackExchangers?