Before Update trigger is also firing on insert, why?

[dean warren]

I have three tables, A, B and C where A is the parent of B, B is A's child. C is the history of B.

Below is a diagram showing the relationships and multiplicity (not both relationships are identifying).

1 0..*
A ------- B
| 1
|
| 0..*
C

The insert statement for B is of the form
insert into B ... on duplicate key update ...

There is a trigger in B such that before an update it copies the current row on B to C i.e. C is contains the history of B.

The above works create when there is no A. When A relationship is added I find that the trigger is fired twice, both on insert into B and update on B - why?

Thanks in advance.