Other than the HDMI cable, was there any other changes you made to how your system was connected? Is the receiver's power cable plugged directly into the wall or a power bar? Or did you plug the power cable into your cable box? If you plugged it into the cable box, the cable box will automatically power on whatever is plugged into it.

OR

You most likely have Bravia Theater Sync enabled. Enabling this feature will cause the receiver to power on as well. Since both your TV and receiver are Sony. I would recommend disabling the Bravia Theater Sync. Since you have the Harmony, you don't need that feature.