It happens to me too!! It's strangely satisfying to see that I'm not the only one with this issue, which is rare to find and hard to describe (so, very difficult to google).
It's been happenning ever since I bought it, a few months ago, and after dealing with it for some time I've had time to analyze the problem. It's not that it launches apps, but it seems to be performing taps on the screen while the phone is locked. I say this because in case I lock the phone inside an app, say, Twitter, just to give an example, instead of while being in the home screen, I find that I've misteriously retweeted something after unlocking via fingerprint. And if I was on the home screen before locking, I find that I've launched an app before unlocking, even though that would seem impossible.
So the behaviour would be as if the phone was buffering screen taps while locked, and performing them right after locking. I tried several times to touch on purpose somewhere in the screen and see if the app that was in that point gets launched, but to no avail.
I'm starting to suspect it's because of the launcher, 'cause I'm using Nova Launcher too. Will try to change it later to see if it helps.
The phone is the S6 non edge, and the OS is Android 5.0.2, Samsung carrier-free ROM, not rooted.