Well, that was a surprise. And yet, looking at the past two centuries of Chechen history, I wonder why I am surprised at all. Conquered by the Russians in 1813, the Chechens revolted at every opportunity. They revolted in 1834. They revolved in 1877. They revolted in 1905, 1917, 1930, and the early 1940s. Chechnya was ethnically cleansed by Stalin in 1944. Chechens were allowed to return to Chechnya under Khrushchev in 1956, but under Soviet "Russification" policies intended to destroy Chechen culture. The Chechens revolted again in 1991. The most recent Chechen rebellion ended with a Russian assault on the Chechen capital of Grozny, with thousands of civilian casualties.
These are people who have never known peace. The motivations of the Tsarnaev brothers have yet to be studied, but I cannot see how the children of Chechnya could be anything war-traumatized. The surprise, perhaps, is that there have not been more international terrorists from Chechnya.