Why did Casey Stoner retire?

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Tags

1. #guard
2. #motorcycle
3. #after
4. #stoner
5. #leading
6. #motogp
7. #suffered
8. #retired
9. #barrier
10. #their
11. #years
12. #forced
13. #announced
14. #racing
15. #malfunction
16. #throttle
17. #three
18. #return
19. #violent
20. #which
21. #casey
22. #crash
23. #march