How to find the reason of Fragmentation ?

Read

http://www.practicalsqldba.com/2012/04/sql-server-index-fragmentation.html

http://blog.sqlauthority.com/2010/01/12/sql-server-fragmentation-detect-fragmentation-and-eliminate-fragmentation/

http://www.mssqltips.com/sql-server-tip-category/39/fragmentation-and-index-maintenance/

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Many Thanks & Best Regards, Hua Min