Other than the ol' trigger-firing-itself-due-to-update-inside-trigger-ploy, 2 other questions:
1) any OTHER triggers on this table?
2) is the join column a primary key or unique index value? Looks like cartesian product if not.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service