Phone just self rebooted. When it came back on it had reverted to a previous wallpaper.

So my phone (HTC Evo 4G) just rebooted itself. After the Sprint load screens it showed the correct wallpaper for a second. Then the screen blinked and it was the previous wallpaper, which it is still displaying. I just changed my wallpaper yesterday. Why would it be displaying a previous wallpaper?