Assembly Build posting to COGS?

I am seeing a curious G/L impact on one of our assembly build transactions.

In addition to debiting the parent asset account and crediting the components' asset accounts, the assembly build transaction also debited COGS and credited the parent asset account for the amount of the build.

I.e. the following transactions occurred:

DR Parent Inventory Asset Account
   CR Component Inventory Asset Account(s)
DR COGS
   CR Parent Inventory Asset Account

This only happened on one instance of an assembly build (other assembly builds did not have the COGS transaction).

Is there a reason why this might have happened? Was it the way the assembly build itself was transacted, or was it due to the way the parent assembly part was set up? How do I prevent it from happening again?