I recall in the early days of Jelly Bean roll out quite a number of you, presumably those who had their Screen Lock set to None like me, reported an annoying occurrence where when you pressed the Home button to wake the phone, doing so would close previously opened apps. My S3 started doing the same recently after I disabled some apps/services from launching. Trouble is I don't know which particular items would trigger this so I did a Applications Manager | (left soft key) | Reset App Preferences and this fixed it. If this doesn't fix it for you, try powering down the phone and restarting. Post back your findings if you have this problem and whether this step fixes it for you.