Hi, & Welcome to the Forums!
This can be caused by a couple of things. Sometimes it's a buggy background program that isn't shutting down properly and it forces the machine to reboot instead of shutting down as it's supposed to.
Another one can be a change to the BIOS Power Management Settings, and it's one that you don't have to physically change it seems as folks have reported this one changing for no apparent reason. I take it you're using the normal way to shut down your computer, Start, click or hover over the ">" in the lower right corner, and click on Shutdown right? There are a few ways to shutdown these days so I have to be sure you don't have a quickie shortcut script on the desktop or if you're using a gadget for it.
If you don't have any custom BOIS settings, I suggest you try this first. Boot to the BIOS and select "Load Default Settings" and then "Exit Saving Changes", then restart and see if that changed this behavior. With Dell I believe it's the Delete key to get to the BIOS, or when your computer restarts it should say on a quick screen something like "Setup = Del" or similar. Whatever it says, that's the key you need to tap on restart to enter the BIOS.
Try this and report back please.