Hi!  I'm just pondering your question.  It's so tough to understand what's going on without seeing the dogs.  MOST dog-to-dog "aggression" is just noise and posturing, or sometimes playing that looks like fighting to us.  Mine only got into a real squabble once.  For the life of me I can't remember what happened but I was able to break them up verbally and there was no damage.  Mostly, Maddie is a submissive dog and Jack is a dominant but tolerant one.   So Maddie will get on Jack's nerves, Jack grumbles and walks away, and every once in a while he yells at her loudly and she backs down.  They DO play-fight very loudly sometimes.  My experience is that if you can see all the teeth (back and front) and they mutually stop on their own, it's playing no matter how bad it sounds.   And most especially, if they "shake it off" with the wet-dog type shake when they are done, it means that all is well (from their point of view).   

Do you have to break the dogs up, or do they stop on their own?  If they stop on their own and there are no injuries, I'd let it go for a bit and see what happens.  My guess is that Ziggy is over-running his "puppy license" and Baxter is getting annoyed.  Basically, until dogs hit adolescence (as you probably know) most adults give them plenty of leeway to be rude little brats and only correct the worse transgressions.  Once the pups hit adolescence or early adulthood, the other adult dogs expect them to start shaping up and minding their manners.  If Baxter is a laid-back guy, he might have let Ziggy get away with it longer.  Now Ziggy may be pushing his luck and Baxter is not ok with that any more.   A few loud arguments between them will probably sort themselves out faster than if you were to intervene.

If, however, neither backs down, or they draw blood, or one yips or squeals and tries to leave and the other continues to pursue, then it is time to intervene.  If any of those things happen, then I'd implement Nothing In Life is Free with both dogs and give more structure to their free time in the house.  I'd also work one-on-one with each of them for short times at least a few days a week;  I know you do agility so you probably already do that.  

Like I said, it can be hard to tell.  When mine play they sound like they are killing each other.   Hope that helps.

Thanks so much for the pointers! I'll take all I can get. I never noticed how much I bend over. You're right, I do think it's because I think he can't see my hands. I will make it a point to stand up straight!

I think he was more stressed about the day than he let on. He acted like a goofball, but he was throwing up and had diarrhea the next day. He also didn't sleep the whole trial, even when I covered his crate up. This was sort of a crazy event for his first trial. I signed up because a lot of the people in my agility class said they were going, but then they flaked out...

I need to look into UKC. Right now he's registered with CPE and TDAA, which from what I've heard are smaller, more relaxed events.