What's actually restarting on my phone?

[WarnerYoung]

I'm just curious. I have an AT&T Galaxy Note 4 with Marshmallow (6.0.1) on it, though I'm on Cricket. Every once in a while (maybe 1 or 2 weeks), things slow down tremendously, and then the phone acts like it's rebooted. It goes to the screen where I have to first enter my password (using fingerprint security), with the message "Your phone has restarted".

But if I check in Settings->About Device->Status, the Up Time counter hasn't reset to zero. Which means it wasn't the entire phone that restarted. So what, exactly, DID restart? The launcher (I'm using Nova Launcher)? Some special part of the Android OS?

And while I'm at it, is there any way for me to figure out why this is happening? If I'm doing something wrong, or if there's a misbehaving app that's causing this, I might be able to change the behavior.

Thanks in advance for any and all answers!

 

[SpookDroid]

I know that there's a way to see a log of events that have happened on your phone and which process triggered them but I don't know how to access them (they're not a simple user setting, you have to use other tools IIRC).
But the up-time counter, if I'm not mistaken, only reboots when the device is powered off, not when it's restarted.