Try swapping the connections on the back of the HR21-700 to see if the "other" tuner starts misbehaving. Move the BBCs, not the coax into the BBCs. If it does move to the "other" tuner, then try to swap the coax going into the BBCs and see if it then stays on the same tuner.
You may have a BBC that has stopped functioning properly.
You may have a BBC that has stopped functioning properly.