Read Committed Snapshot Isolation blocking during table modification
Are you doing online rebuilds of this index at the same time as the transaction?
I have RCSI block if there is an index rebuild simultaneous with a large or long running transaction.
Paul Randal has a great explanation here: http://www.sqlskills.com/blogs/paul/a-sql-server-dba-myth-a-day-830-unicorns-rainbows-and-online-index-operations/