Andrew, I have seen some dogs that display a similar response to what you described, as though they are all the sudden fearful. Sometimes, it almost seems as though the coming of age pup KNOWS that it is crossing a line, and is expecting the discipline, but just HAS to try it anyway just in case...
structure is instinctive in a dog, so this is not the same as someone claiming a dog "knows when it did wrong" or anythign that would suggest they remember tearing up the couch 5 hours ago.
When a dog is challenging for rank, the response is either acceptance or a returning challenge that is stronger than theirs. They test rank because that keeps the pack strong, and keeps the strongest one in charge. This increases every members' chance for survival. When he challenges you, it's either because he feels cornered, or because he has established (in subtle ways no doubt) that he is actually in charge, and you are disrespecting his position.
Take the advice given with the leadership exercises, visit the vet to make sure, but also:
try not to "hover" over him to make sure its not a fear based "cornered" response.
Also - how is he fed? Free choice? Scheduled meal times? Does he have a time limit?
Where is he fed? In the middle of a bunch of people walking around the kitchen? Does he have the room to himself when he eats? Have you ever taken the food away while he was eating it to "teach" him? Have you taken bones or other things away to try and teach him not to bite you when you take them?
The reason I ask is because eating and feeding times are a huge thing to dogs.
Whoever controls the food is in charge. Free feeding allows the dog to decide when to eat, and how much. Not good for health reasons, but also because it promotes aggression and dominance.
When the dog is fed, it should have a time limit, or until he walks away from the food. I usually give 10 mins (but it's usually gone in 2) If the dog still has it's nose in the bowl, I let it finish, but the minute it leaves, it's gone.
Don't leave bowls down, pick them up. With some dogs I have even had to pick up water bowls to prevent guarding.
A dog should be able to eat in peace, no matter what. Yes, you ideally should be able to pick up the food bowl while they are eating. But there is no reason. You already gave the food to the dog, taking it away, even to add more food or give it right back is bullying the dog. If you do mess with their food, it is hands down a great way to destroy trust with the dog. With a high drive dog, it can cause a bigger response than with a dog that doesn't really care. I recommend feeding in the crate, always. That way, the dog eats in peace, no kids (yours or visiting) can get in the way or get nipped for being to close, and the dog doesn't ever begin to resource guard (usually) because there is no need.
Sorry for the long post. just covering something that wasn't yet mentioned.
When a flower doesn't bloom, you fix the environment in which it grows, not the flower.