Horses who get drilled doing the same thing every time can get bored. Some horses are just fine, others need more stimulation. It's kind of like being stuck in 5th grade. You know all the information, yet you are still being quizzed and drilled on stuff you know really well. Perhaps its time you move onto something else. It doesnt have to be dramatically difficult, but it just needs to be something different. Ground poles, obsticles through cones, trail exersizes (opening gates, sidepassing poles, backing through shapes, dragging things, etc) tricks, or perhaps even liberty play. The list is endless. Im thinking maybe he is trying to tell you he is bored with his work regimen.
Other than that, it could be sparked by his frustration of y our lack of clarity. Most riders are not clear enough, and some horses don't mind, others get really bothered by it. So as I said before, stick to something really pure and simple, something you know you can be very clear for him to follow.
Could also be a behavioral thing as well, him thinking he can be naughty to get his way. You could go about giving him a harder job to do once he has started to act up so that it is just easier in his mind to be with you and be quiet and obedient. Like serpentines, figure eights, etc.
Lots of possibilites. Just keep exploring with him, think outside the box. He has a pattern, you have a pattern, do something different to break those patterns.