Diagnosis
The HDMI CEC and/or LAN Control features are turned on, and a signal from a connected device is waking it up.
How to Fix It
Press the menu button on the remote. Go to System -> System Setup. Set HDMI Control to Off. Then, go to LAN Control and set it to Off as well. This severs the automatic wake-up signals.
If That Didn't Work
Check the Auto Power On setting in the same System menu. Ensure it's set to Off. Also, physically disconnect the HDMI cable from the projector when not in use to eliminate any phantom power signals.
Heads Up
If you use a smart home system (like Control4) that integrated with the projector, turning off LAN Control will break that integration. You'll need to configure the system to not send spurious wake commands instead.