Apple Mac OS X has a flaw that’s been in several versions of the operating system, including the latest public release of Yosemite. Programs that have been quit will still show up as running in the background when you view the Force Quit Applications window (accessible from the Apple menu or by pressing alt/option + command + esc).
In the example shown below, several programs are running, as is indicated by the black dot under the program icon in the dock. The Preview program doesn’t appear to be running since it has no black dot under it. Yet, it is listed in the Force Quit Applications window.
This problem seems to happen on a variety of Apple hardware platforms, and with various versions of OS X. For some reason, it’s still not fixed.
Workaround / Solution
At present, the best workaround seems to be periodically checking the Force Quit Applications window, or restarting the computer to ensure all processes are completely stopped and that memory has been properly released back to the system.