That last part makes sense...

I'm thinking an "IF" condition on transaction_type may be a step in the right direction (?):

USE [TrackIT]

GO

CREATE TRIGGER [dbo].[trDecrementBulkPurchases]

ON [dbo].[tblTransactions]

AFTER INSERT

AS

BEGIN

IF Transaction_Type = 'From Bulk Assignment'

UPDATE [TrackIT].[dbo].[tblBulkPurchases]

SET [Quantity_Remaining] = [Quantity_Remaining] - 1

WHERE PO_Number IN (SELECT [PO_Number] FROM [tblTransactions])

END IF

END

GO

-- there are some syntax issues of course