One of our custom apps caused such a thing on another handheld OS. Think over what apps you installed recently and work backwards.
This now happens almost 100% of the time. At the end of the day when I power off the phone, sometime within the next 3 to 10 minutes it powers itself back up again. A second power off then results in the phone staying off for the night.
Android 2.3.3 System ver. 4.5.605.MB810