Broken Trek Session 88

My friend was riding his Trek Session 88 for couple of seasons. Trek Session is a very lightweight frame. The whole bike with Boxxer and average components had a weight less than 17kg (37.5lbs). Session was the lightest downhill frame on the market in the year of manufaturing. Maybe this lightness caused some strength problems...

During season 2012 we were visiting Finnish bike parks. This case had happened with my friend at Sappee bike park. He was riding his bike and he was not experiencing any problems even sending rather big gaps.

After hucking one small gap a little bit to flat Trek Session had lost its chainstay...

We were very confused. Weight of the owner of the bike was less than 70kg (140lbs), and he was rather experienced rider.
As I know, this wasn't the only case with broken chainstay of Trek bikes.
This frame was bought used, that's why my friend didn't have any documents of it. If you don't have documents or invoice it's very hard to get warranty replacement in our country. He had decided to get the chainstay welded by rather famous russian frame welder. Just look at this pic:

The chainstay was strengthened with additional alloy gussets.

 After the frame was repaired, my friend was riding his Trek Sesson 88 for couple of months and the chainstay is still in its place!