the seat tube was too big for the seat post, and so it was just rocking back and forth on the clamp. Broke at the front of the seat tube/top tube juncture as demonstrated by how dirty the fracture surface is there. It's pretty common for aluminum bikes with a lot of seatpost sticking out to have a failure at that point. I have seen fisher mountain bikes with that failure there. But those have a frame warranty.

I would have thought this frame would be made with one of the aluminum series that doesn't require post-weld heat treatment. I thought 6061 wasn't used much in bikes.