Backup Technician Assignment Failed or Overridden?

We have several business rules in effect, dictating that tickets with a certain category or subcategory be assigned to a specific technician, so that category X tickets go to tech A, category Y goes to tech B, and category Z goes to tech C. We also use the Scheduler to allow everyone to see when a particular tech is out, and for that tech to specify a backup so that all tickets will be assigned to someone else. I've seen occasions where this didn't seem to work, but didn't look into it. Today, though, it came back to bite us as a top-priority ticket created yesterday was not handled until today. There is a business rule we have as a safety net that says if a ticket does not have a category of X, Y, or Z, it should be assigned to tech D. None of the rules have the cascade feature turned on, and this safety net rule is below all the others. However, on February 3, tech D marked himself as being out on February 16, and all his tickets should be assigned to tech A. Yesterday, Feb 16, two tickets were created that were not category X, Y, or Z, and so should normally be assigned to tech D, and in fact it was, even though that is not what I would have expected to happen due to the Scheduler setting. History on the tickets show changes to various settings once they got worked, but not the technician, and the tech confirms that he saw them as assigned to him when he came in today.

So my question is two-fold. First, why did this happen? Do business rules take priority over other settings? Is there any way to determine if this was some sort of system error or lapse in handling? Secondly, if so, is there a better way to handle assignments? We have a small group of 6 technicians who each have their own specialty, so it really does work to use business rules based on category. However, if that conflicts with the way the system works, it's not very helpful and is actually a negative when someone is out.