Direct observation of a simultaneous break on both ends:

Three years ago I was flying in the front seat of a Grob 103 (nose tow hook) with an instructor for a spring checkout. He wanted me to try a slack rope recovery method that was very different from normal (and later we agreed was not the right approach). When I did the maneuver, we had a spectacular rope break...I could see the rope break simultaneously at the tow plane end and the glider end and fall away.

This has an aerodynamically interesting ending.

The instructor could hear the broken rope segment on our end making noise against the fuselage. We had plenty of height, so we let the tow plane land, and moved over the runway to drop our short length of rope so the tow ring, etc could be recovered easily. When I pulled the nose tow release to drop the rope, a second or two later I noticed something at the open canopy window to my left. There was a foot or less of rope flapping around just inside the cockpit, so I pulled gently on it, and the entire three feet or so of the broken rope segment with the tow ring on the end came in through the window. It was trapped in some low pressure region against the fuselage which was connected to the flow through the window!

I agree with the wave/propagation idea.