Yes, it happened to me when free ram goes very low e.g. Asphalt8; you can't exit with back button, had to press home.
Could be related to poor ram management (in our stock ROMs). Maybe some "service" is shutting down which is related to back button function.. idk. If you're getting this with Opera/FB means, quite poor ram management I think.. Maybe lot of bloatware is being run in the background eating ram?
*Just monitor the amount of free ram with CoolTool app (at Google play), to see whether it goes very low when it happens. Cooltool itself may shutdown due to low ram..
*Try Greenify app to hibernate apps, free more ram.
*See whether anything could be done to keep more free ram, from this thread: http://forum.xda-developers.com/show....php?t=2751559
Edit: saw your app list. I would ditch half of the list if rooted
. Chrome and FB are the first to go :P