I've seen this twice from the glider. The second time caught on a GoPro.

The failure mode appears to be recovering from slack in a nose up condition..
The cause is a combination of physics, glider pilot training, and towing in sporty conditions.

My first experience was at altitude behind a Pawnee. From the glider, the stick was full forward, little elevator authority, rope tight, and pilot leaning forward. Glider nose was slowly lowering so the situation was converging. Talking on the ground, the tow pilot never lost elevator authority or felt the need to release his end. The lesson learned was the wrong one with little understanding. That this was strange, but safe and recoverable event.

The second experience was just after launch behind a CallAir. Similar story from the glider, but a much different story from the tow end. Loss of elevator authority, ground getting close, but situation improving. If I had had any clue that there was a problem at the other end of the rope, I would have released, but from the first experience, I saw things as ok and improving. Thankfully, it turned out ok, but with a much different lesson and understanding on the second try.

Having had some time to think about it. I think an understanding of kiting should be required for anybody towing with a CG hook. Actually trying to train it at altitude is not a good idea because of loss of sight of the tow plane. Remembering not to be nose up on slack recovery is. If you get to this mode, just quickly release and go again.

I've since put a nose hook on my glider. This was a non-trivial exercise, but the opportunity presented itself and since I tow in sporty conditions, it seemed worth it. Given an understanding of the problem, this is kind of belt and suspenders, so it probably should not be a requirement, but the training is a must.