Just a thought,
The only way I can see that happening is someone went into the Incident Tree and updated the Assignment manually. I don't think this is listed in the Audit Trail, just when actions are used does it show in the Audit Trail....
I suppose if the Analyst assigning the Incident assigned it incorrectly in the first place then as you say changed the assigment in the history thinking it would re assign, that would explain it. I need to make the assignments read only when viewed from the history to stop them doing this again!