Error deleting more than 1 row
SQL Server does not support FOR EACH ROW clause in trigger.
So, I think that query in a trigger falls down with an error like this

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, &lt;, &lt;= , >, >= or when the subquery is used as an expression.

Maybe replacement of "=" by "IN" will help.