What switches off the screen?

[Android Central Question]

I have an old tablet - Nexus 10, running Lollipop 5.1.1. I use it to display a calendar app (DakBoard), under a restricted account, but after about half a day of running fine it either switches off the screen, or reverts to the default user. I've never noticed the moment when it happens, so don't know if there's a specific event that triggers that (and I have the notifications disabled, so it's not that). Any ideas what settings I should be looking at, or generally how to troubleshoot this?
Many thanks,
Nikola