Inside a trigger you have access to the inserted and deleted pseudo-tables which contain the rows that were affected by the operation that fired the trigger. You need to change the trigger so that the update joins or filters based on the inserted table so that it only updates rows that were affected by the insert.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability