What keeps waking my Mac up from sleep?
As for what:

The message Kernel Assertions: 0x10c=USB,BT-HID,MAGICWAKE says it.

Also the ExternalMedia is set to wake it up over your USBbluetooth. connection.

So you have a device (BT keyboard) that does that.

As for why:

You BT track pad wakes up (do you have a Cat?) other BT devices interference, low battery... see this for more.

To test do "Allow Bluetooth devices to wake this computer" OFF.

If you have the Magicwake app installed, remove it.