But why did it turn the 976 off if it was not demanded? Did you turn off some other connected component or maybe one of your connected devices went to sleep and shut down the 976 with it. The possibility that you could have many connected devices and each of them could cause each other to do things not demanded makes it hard to troubleshoot potential issues with equipment.
I do know that new TVs, like my Vizio, will auto power off, with a default setting of 10 minutes, when idle.
I disabled that setting in my TV.
This shouldn't happen if the CEC settings on the 976, are disabled.