Why is cold water coming out of my HW OUT-TAKE pipe?
If this happens when all of the faucets in the house are closed, it would have to be a mixing valve of some sort.    

If any single spout faucets are open (perhaps you had them open to bleed air out of the lines), they will allow water to move between the hot and cold sides if both valves are open (normally, this doesn't happen, because both sides are pressurized equally, so all the water comes out the spout).

A smaller possibility would be a recirculating pump that is meant to make sure there's ready hot water at a distant fixture -- these usually work by pumping water from the hot pipe near the fixture back into the cold near the tank.   It's possible that water could move backward through that in your situation.

I would check that all valves are closed (including making sure the clothes washer is off, since it could be trying to mix hot and cold) and then test again.