This is called Automatic Termination and it's standard behaviour on Lion. Beginn...

DrJokepu on Feb 17, 2012 | parent | context | favorite | on: New Features in Xcode 4.3

This is called Automatic Termination and it's standard behaviour on Lion. Beginning OS X 10.7, the OS automatically suspends processes that are "safe to close" and opt in to Automatic Termination when certain conditions are met. It could happen even to TextEdit or any other app, not just Xcode.