Yay, thank you so much for the post!
I'm using a LG-D855 16gb version of the G3 - it's a European version and its carrier free. I have the latest update - v10g.
When I had apps open and closed them so I can go back to the home screen the home screen just started because it closed itself before. Its really strange and I could even understand it but I used one single app at a time and it still did the same! Clock, calendar, or the app of this forum. And as you can see I didn't had a lot of background processes in the background.
Since then I switched to Nova launcher and it seems it's not restarting (as often - at least) like the stock launcher.
But I would like to use the stock launcher without any problem as it supposed to work.
Apps are still tending to close almost immediately when I put them to the background - not 100% of them, but most of them. It can be a little bit annoying.
Bump: I still have the issue - am I the only one without any solution? I tried Nova - it was a little bit better only once or twice happened.
Today I gave another shot to the stock launcher: happened 3 or 4 times within a few hours - not using any big apps like games or anything, only chrome or messaging.
Do you guys have any suggestions? Thank you in advance.

(Still love the phone but would be awesome to figure out what's happening
