In Lightroom with Photoshop NOT open, I select an image and do a Command-E, the Photoshop splash screen comes up and then the image I selected to open opens as it should. However, if I go back to Lightroom (with Photoshop now open as well), select a different image and do a Command-E, it opens in the background. That is, I have to manually move to Photoshop to see the image I selected from Lightroom, Photoshop doesn't come to the front, Lightroom stays there.
This works fine for me under Windows: LR 5.5, PS CC 2014.
It might help if you told us what versions of Lightroom and Photoshop you're using on your Mac. Maybe the OSX version, too.
Yes, sorry, LR 5.5, PS CC, Mountain Lion 10.8.5, 2008 8-core Powermac, 16gb RAM.