Automatic Rollback

I am getting an automatic rollback in a transaction that fails to insert a row, via a trigger, due to a duplicate key. The transaction is automatically rolled back and unless the error correctly handled a 'partial update' occurs. Why in the example below is the transaction automatically rolled back (statements 1 to 3) and is there any way of knowing what errors generate an auto rollback. For example if the duplicate key error occurs on the insert into table B below, and not the triggered update, then no auto rollback occurs.

Begin Tran -- 1
insert into A values ('A1') -- 2 updates OK
insert into B values ('B1') -- 3 updates OK
-- triggers insert into B_Hist -- 4 Fails with Duplicate Key error
insert into C values ('C1') -- 5 updates OK
Commit Tran -- 6 3902 error No corresponding Begin

Thanks.