As the title says, why?
90% of my racing is clean, with 10% spitting the dummy out and retaliating on people who have caused me grief.
Although I didnt realise you could get banned for that if thats the case, should have a disclaimer!
Another gamertag iMuggins has the same issue.
How do you tell what got you banned? Look to your behavior.
You (and a Friend) went overboard in causing wrecks, targeting people on several tracks. Driving the opposite direction on the race course, waiting to crash people going by is unsportsmanlike behavior and a violation of the Xbox Live Code of Conduct.
The first ban for such behavior is seven days (168 hours from the time Turn 10 hit the button). A second ban would be 30 days - 720 hours. Third is permanent. Neither bans nor the enforcement of the regulations by Turn 10 are discussed on these forums.