I had a bicycle that I was riding one night and thought that I had got a flat in the rear wheel as the bike suddenly felt squirmy. I got off the bike and felt the rear tire and it was hard. SO I figured maybe loose hub cones or broken axle. I pulled on the wheel. Wheel felt fine but moved a bit. Shone my headlight on the frame and saw this. Both seatstays had torn off the seattube. Sometimes bike frames just wear out.
Cheers