Mountain Lion auto closing apps
Quoting from this typically comprehensive Ars Technica article:
http://arstechnica.com/apple/2011/07/mac-os-x-10-7/8/


  Lion will quit your running applications behind your back if it decides it needs the resources, and if you don't appear to be using them. The heuristic for determining whether an application is "in use" is very conservative: it must not be the active application, it must have no visible, non-minimized windows—and, of course, it must explicitly support Automatic Termination.
  
  Automatic Termination works hand-in-hand with autosave. Any
  application that supports Automatic Termination should also support
  autosave and document restore. Since only applications with no visible
  windows are eligible for Automatic Termination, and since by default
  the Dock does not indicate whether or not an application is running,
  the user might not even notice when an application is automatically
  terminated by the system. No dialog boxes will ask about unsaved
  changes, and when the user clicks on the application in the Dock to
  reactivate it, it should relaunch and appear exactly as it did before
  it was terminated.