Folks, some of my might recall my recent disaster with this tripod and a smashed 24-70. I have solved the mystery as to why this occured, and while doing some tests today I discovered it. Located below the metallic latch of this head plate there is a screw with washer that tightens using an Allen key. It is vertically orientated above the horizontal degree arc. Mine was significantly loosened, such that it rendered the latch ineffective (unfortunately the washer configuration prevents this from being obvious that the screw is loose). I would urge you if using this head to check this screw is tightened so that you don't have any problems, and to check it regularly.
I hope it saves you the pain I had.