Could also happen when you set your evic down or in your pocket. If you drop it/shake it/rattle it hard enough, it will compress the spring and break contact from your battery to the control head for a split second, which is the same as turning it off.
If it is just going into sleep mode and you don't have to power it on, I honestly don't know what to say. I have the latest firmware, and my evic hasn't done that to me since I got it back in June.