Sounds like it may have been "screen saver" esq mode engaging turning off boinc. I had that with my laptop and tablet, and now you've told your pc not to do that because I'm not just running something here all the time, I am running something here all the time. Very minor difference.
I said "screen saver" as you can crunch in screen saver mode, and because you don't need a screen saver to appear for a pc to be "sleeping". However I am not a IT professional so these are just my thoughts and opinions.