I had that harness-as-anchor thing happen to me too when my harness broke off earlier this year, but in my case it was a good thing as it allowed me to catch up with my runaway gear. The wind was cross-off, so at first when my harness broke off I thought that it was definitely going to be bye-bye to my new kite and bar...
However, the kite quickly crashed in the water and then the harness acted like a drag anchor in the water behind it, slowing down the escaping kite. The harness was perhaps 25 meters downwind of me, submerged just undeneath the surface of the water as it was getting dragged downwind by the kite. I saw that it wasn't moving very fast, so I figured I had this one chance to get hold of the harness & bar combo before the kite and everything sailed out to sea. So, lying on top of my directional, I made a mad paddling dash for the harness. It took a minute or so but eventually I caught up with it and could secure the kite
That was a great feeling, let me tell you. With the wind blowing out to sea, there was no way I'd gotten that kite & bar back otherwise.
As for what happened to the harness, I obviously took a close look at it once I had self-rescued everything back to the beach. It turned out to be the stitching that had given away. There had been no indications beforehand that the harness was about to break (the stitching that ripped was between the outer shell and the inside lining, so to speak, so any pre-existing damage wouldn't have been visible). The harness was a Dakine Pyro 2008, used for about a year. From what I have read later, other Pyro harnesses from that year have failed in similar manners. Damned shame, cause that model & year is the most comfortable and light harness that I've found.
The harness broke when I made a bottom turn on a small wave. There was obviously some force on the harness at that point, but it must have been on the verge or breaking for some time before that.
The windspeed was only about 22-23 knots. Had it been stronger, I doubt that I'd been able to catch up with the kite in the water.