Ok, I tried various camera settings(auto zones, creative zones, Al Servo, Al focus, One shot, manual & auto focus, IS on & off, lens changes) and no change. I do have some details though.
A) Setup: Testing with 40d. Metz off-camera, Yongnuo on-camera(mounted via Phottix II transmitter).
1) With a push of any of the camera's top adjustment buttons(ISO, WB, AF/drive) the Metz flash will fire once. It sends a signal to fire for some reason.
2) When I properly set the Metz to manual mode(off camera), it does the rapid firing as described. However, when I set the Metz to ettl mode, it only fires once with the half-push of the shutter button.
3) When the Metz begins the rapid firing I can turn off the Phottix transmitter and the Metz will stop. If I quickly turn the transmitter back on, the Metz will continue with the rapid firing. If I wait a few seconds before turning the transmitter back on, the Metz is idle with such. It's as if a short command is being sent. If only interupted very briefly the Metz will continue with the rapid firing. If interupted for a few seconds, as if the commad finished, the Metz is logically idle upon turning the transmitter back on.
4) If I turn the Yongnuo flash off, even though the Photix II transmitter is on, the Metz receives no signal. It should but doesn't.
If I then remove the Yongnuo flash, leaving only the Phottix II transmitter in the hot shoe, the off-camera Metz works properly as previously described.
5) This isn't new but only a reminder. If I reverse flashes (Yongnuo off-camera, Metz on-camera) everything works properly.
B) Setup: Testing with 20d. Everything works properly. I can swap out flashes, make adjustements, etc and everything fires as expected. No surprises.
I'm stumped. It's not a real issue at the moment, as I prefer the Metz on-camera, but could become an issue with future setups.