Sometimes I'll be playing a game, then go to check something on Twitter, and then open the game back up and it's restarted.
Found a neat little trick to help:
1. Go to Settings > Additional Settings > Recent Tasks Manager
2. Toggle apps that you don't want restarted all the time to On
And that's it really, the phone is fine for lighter apps, but when it comes to some games it doesn't handle multitasking too well, but if you "lock" the game with the above method then it won't close it as often.
Also might be worth going into the Phone Manager app > Settings > Self Check > Off. This also helps.