Why did my InDesign preview mode background turn black?

I know that you can change the preview mode background color in Preferences>Guides and Pasteboards. When I was working on a document, without going into preferences, my preview mode (shortcut: W) randomly turned black. I am assuming I pressed some keyboard shortcut accidentally or it was a glitch? I went into the Preferences to change the color back to gray, and the background was now set as Black, even though I did not change it in Preferences originally. Anyone know what's up?