He looked like he wanted to flex his neck down well during those canter circles and your hands were blocking him.
I think you pinpointed part of the issue (hands that do not follow). Now that you are aware of it, be hyper aware of it and fix it. I'd suggesting lengthening your reins just a bit and bringing your elbows back the same amount. As our joints approach full extension, their flexibility and suppleness disappears PDQ.