Can the wallpaper setting of my ZTE Obsidian accidently change from a bug in the OS?

[Ianiscurious]

Or does someone have to manually change the settings? I left my Android OS, ZTE Obsidian unattended and when I returned to the phone and turned it on, the stock wallpaper I had it set to had somehow changed to a pic someone had sent to me and was saved in the gallery of the phone. According to http://www.gsmarena.com/zte_obsidian-7481.php the obsidian has an Android OS, v5.1 (Lollipop). I searched their site for possible bugs but could find no reference thereto. Access to the phone had always been prevented unless and until my pin was entered. This lock function remained operable when I returned to the phone. The pin remained the same for me to gain access. I next easily reset the wallpaper default back to the purple sunset. How could that wallpaper have changed without someone changing the wallpaper setting? is it possible for the settings to have some kind of bug that might have caused this or did someone, somehow get in and change it ? Has anyone else encountered anything like this?

THANKS !