The problem:  

If your source is publishing to a topic with more than 1 partition, we cannot guarantee that sinks will consume a document's changes in the order in which they occur.  Create could go to partition 0, update to 1, and delete to 2.  A slow sink on 0 and a fast sink on 2 means that we could get a delete before a create.

If we could assign the partition based on the documentId, we could ensure that a given document is processed in the right order.