Hello,
this problem has been bugging for some time now and I can't figure out what's causing it. I have a Dell XPS 9560 and since I upgraded to Fedora 28, most of the times when I power off the laptop it shuts down, but then after about 2 seconds starts up again by itself. Once I'm back at the login screen and power it off from there it shuts down properly although it will show the Fedora logo for about 2 minutes before turning off. It makes no difference if I use Gnome's power off button or use the shutdown command in the terminal. On Fedora 27 I had no such problems.
I have the feeling it might be related to the laptop being suspended at some point because if I use it for a while without going into sleep mode in between booting and turning it off it will shutdown properly. I'm not 100% sure that's always the case though. I don't use hibernation. I couldn't find any suspicious messages in the journal although I'm not sure what I'm looking for either. Maybe it also has something to do with USB management since on startup I get the warning 'xHCI host controller not responding, assume dead' (USB ports are working fine though).
Does anybody have some advice how to solve or debug this? Thanks in advance!