The statement has been terminated
Insert into Employee_History
  Values(@EmployeeID,@Employeename,@SSN,@Phonenumber,@JoinDate,@OperationHistory,getdate())


Make sure the column order of the Employee_History table matches the order of the variables listed.  The best practice is to list column names on INSERT statements.  

The other possible cause is that the EmployeeName column in the Employee_History is shorter than 100 characters.

Importantly, as suggested in the comments, triggers fire once per statement instead of once per row so must be coded to handle multi-row inserts.  The history table insert should be refactored as INSERT INTO Employee_History(column-list) SELECT column-list FROM inserted  Not related to this error, though.

Also, there are a number of other errors in the trigger.  An UPDATE statement can update multiple columns yet the trigger code identifies only one the potentially changed columns.  The UPDATED function in an UPDATE trigger does not indicate the value has changed, only that it specified in the SET clause of the UPDATE statement.  This may cause erroneous messages in the history table.